Statistics: Rewrite statistics backends to use OpenMetrics

The metric subsystem of Prosody has had some shortcomings from
the perspective of the current state-of-the-art in metric
observability.

The OpenMetrics standard [0] is a formalization of the data
model (and serialization format) of the well-known and
widely-used Prometheus [1] software stack.

The previous stats subsystem of Prosody did not map well to that
format (see e.g. [2] and [3]); the key reason is that it was
trying to do too much math on its own ([2]) while lacking
first-class support for "families" of metrics ([3]) and
structured metric metadata (despite the `extra` argument to
metrics, there was no standard way of representing common things
like "tags" or "labels").

Even though OpenMetrics has grown from the Prometheus world of
monitoring, it maps well to other popular monitoring stacks
such as:

- InfluxDB (labels can be mapped to tags and fields as necessary)
- Carbon/Graphite (labels can be attached to the metric name with
  dot-separation)
- StatsD (see graphite when assuming that graphite is used as
  backend, which is the default)

The util.statsd module has been ported to use the OpenMetrics
model as a proof of concept. An implementation which exposes
the util.statistics backend data as Prometheus metrics is
ready for publishing in prosody-modules (most likely as
mod_openmetrics_prometheus to avoid breaking existing 0.11
deployments).

At the same time, the previous measure()-based API had one major
advantage: It is really simple and easy to use without requiring
lots of knowledge about OpenMetrics or similar concepts. For that
reason as well as compatibility with existing code, it is preserved
and may even be extended in the future.

However, code relying on the `stats-updated` event as well as
`get_stats` from `statsmanager` will break because the data
model has changed completely; in case of `stats-updated`, the
code will simply not run (as the event was renamed in order
to avoid conflicts); the `get_stats` function has been removed
completely (so it will cause a traceback when it is attempted
to be used).

Note that the measure_*_event methods have been removed from
the module API. I was unable to find any uses or documentation
and thus deemed they should not be ported. Re-implementation is
possible when necessary.

   [0]: https://openmetrics.io/
   [1]: https://prometheus.io/
   [2]: #959
   [3]: #960

3 files changed, 722 insertions, 209 deletions

diff --git a/util/openmetrics.lua b/util/openmetrics.lua
new file mode 100644
index 00000000..299b36c7
--- /dev/null
+++ b/util/openmetrics.lua
@@ -0,0 +1,308 @@
+--[[
+This module implements a subset of the OpenMetrics Internet Draft version 00.
+
+URL: https://tools.ietf.org/html/draft-richih-opsawg-openmetrics-00
+
+The following metric types are supported:
+
+- Counter
+- Gauge
+- Histogram
+- Summary
+
+It is used by util.statsd and util.statistics to provite the OpenMetrics API.
+
+To understand what this module is about, it is useful to familiarize oneself
+with the terms MetricFamily, Metric, LabelSet, Label and MetricPoint as
+defined in the I-D linked above.
+--]]
+-- metric constructor interface:
+-- metric_ctor(..., family_name, labels, extra)
+
+local time = require "util.time".now;
+local select = select;
+local array = require "util.array";
+local log = require "util.logger".init("util.openmetrics");
+local new_multitable = require "util.multitable".new;
+local iter_multitable = require "util.multitable".iter;
+
+-- BEGIN of Utility: "metric proxy"
+-- This allows to wrap a MetricFamily in a proxy which only provides the
+-- `with_labels` and `with_partial_label` methods. This allows to pre-set one
+-- or more labels on a metric family. This is used in particular via
+-- `with_partial_label` by the moduleapi in order to pre-set the `host` label
+-- on metrics created in non-global modules.
+local metric_proxy_mt = {}
+metric_proxy_mt.__index = metric_proxy_mt
+
+local function new_metric_proxy(metric_family, with_labels_proxy_fun)
+	return {
+		_family = metric_family,
+		with_labels = function(self, ...)
+			return with_labels_proxy_fun(self._family, ...)
+		end;
+		with_partial_label = function(self, label)
+			return new_metric_proxy(self._family, function(family, ...)
+				return family:with_labels(label, ...)
+			end)
+		end
+	}
+end
+
+-- END of Utility: "metric proxy"
+
+local function render_histogram_le(v)
+	if v == 1/0 then
+		-- I-D-00: 4.1.2.2.1:
+		--    Exposers MUST produce output for positive infinity as +Inf.
+		return "+Inf"
+	end
+
+	return string.format("%g", v)
+end
+
+-- BEGIN of generic MetricFamily implementation
+
+local metric_family_mt = {}
+metric_family_mt.__index = metric_family_mt
+
+local function histogram_metric_ctor(orig_ctor, buckets)
+	return function(family_name, labels, extra)
+		return orig_ctor(buckets, family_name, labels, extra)
+	end
+end
+
+local function new_metric_family(backend, type_, family_name, unit, description, label_keys, extra)
+	local metric_ctor = assert(backend[type_], "statistics backend does not support "..type_.." metrics families")
+	local labels = label_keys or {}
+	local user_labels = #labels
+	if type_ == "histogram" then
+		local buckets = extra and extra.buckets
+		if not buckets then
+			error("no buckets given for histogram metric")
+		end
+		buckets = array(buckets)
+		buckets:push(1/0)  -- must have +inf bucket
+
+		metric_ctor = histogram_metric_ctor(metric_ctor, buckets)
+	end
+
+	local data
+	if #labels == 0 then
+		data = metric_ctor(family_name, nil, extra)
+	else
+		data = new_multitable()
+	end
+
+	local mf = {
+		family_name = family_name,
+		data = data,
+		type_ = type_,
+		unit = unit,
+		description = description,
+		user_labels = user_labels,
+		label_keys = labels,
+		extra = extra,
+		_metric_ctor = metric_ctor,
+	}
+	setmetatable(mf, metric_family_mt);
+	return mf
+end
+
+function metric_family_mt:new_metric(labels)
+	return self._metric_ctor(self.family_name, labels, self.extra)
+end
+
+function metric_family_mt:clear()
+	for _, metric in self:iter_metrics() do
+		metric:reset()
+	end
+end
+
+function metric_family_mt:with_labels(...)
+	local count = select('#', ...)
+	if count ~= self.user_labels then
+		error("number of labels passed to with_labels does not match number of label keys")
+	end
+	if count == 0 then
+		return self.data
+	end
+	local metric = self.data:get(...)
+	if not metric then
+		local values = table.pack(...)
+		metric = self:new_metric(values)
+		values[values.n+1] = metric
+		self.data:set(table.unpack(values, 1, values.n+1))
+	end
+	return metric
+end
+
+function metric_family_mt:with_partial_label(label)
+	return new_metric_proxy(self, function (family, ...)
+		return family:with_labels(label, ...)
+	end)
+end
+
+function metric_family_mt:iter_metrics()
+	if #self.label_keys == 0 then
+		local done = false
+		return function()
+			if done then
+				return nil
+			end
+			done = true
+			return {}, self.data
+		end
+	end
+	local searchkeys = {};
+	local nlabels = #self.label_keys
+	for i=1,nlabels do
+		searchkeys[i] = nil;
+	end
+	local it, state = iter_multitable(self.data, table.unpack(searchkeys, 1, nlabels))
+	return function(_s)
+		local label_values = table.pack(it(_s))
+		if label_values.n == 0 then
+			return nil, nil
+		end
+		local metric = label_values[label_values.n]
+		label_values[label_values.n] = nil
+		label_values.n = label_values.n - 1
+		return label_values, metric
+	end, state
+end
+
+-- END of generic MetricFamily implementation
+
+-- BEGIN of MetricRegistry implementation
+
+
+-- Helper to test whether two metrics are "equal".
+local function equal_metric_family(mf1, mf2)
+	if mf1.type_ ~= mf2.type_ then
+		return false
+	end
+	if #mf1.label_keys ~= #mf2.label_keys then
+		return false
+	end
+	-- Ignoring unit here because in general it'll be part of the name anyway
+	-- So either the unit was moved into/out of the name (which is a valid)
+	-- thing to do on an upgrade or we would expect not to see any conflicts
+	-- anyway.
+	--[[
+	if mf1.unit ~= mf2.unit then
+		return false
+	end
+	]]
+	for i, key in ipairs(mf1.label_keys) do
+		if key ~= mf2.label_keys[i] then
+			return false
+		end
+	end
+	return true
+end
+
+-- If the unit is not empty, add it to the full name as per the I-D spec.
+local function compose_name(name, unit)
+	local full_name = name
+	if unit and unit ~= "" then
+		full_name = full_name .. "_" .. unit
+	end
+	-- TODO: prohibit certain suffixes used by metrics if where they may cause
+	-- conflicts
+	return full_name
+end
+
+local metric_registry_mt = {}
+metric_registry_mt.__index = metric_registry_mt
+
+local function new_metric_registry(backend)
+	local reg = {
+		families = {},
+		backend = backend,
+	}
+	setmetatable(reg, metric_registry_mt)
+	return reg
+end
+
+function metric_registry_mt:register_metric_family(name, metric_family)
+	local existing = self.families[name];
+	if existing then
+		if not equal_metric_family(metric_family, existing) then
+			-- We could either be strict about this, or replace the
+			-- existing metric family with the new one.
+			-- Being strict is nice to avoid programming errors /
+			-- conflicts, but causes issues when a new version of a module
+			-- is loaded.
+			--
+			-- We will thus assume that the new metric is the correct one;
+			-- That is probably OK because unless you're reaching down into
+			-- the util.openmetrics or core.statsmanager API, your metric
+			-- name is going to be scoped to `prosody_mod_$modulename`
+			-- anyway and the damage is thus controlled.
+			--
+			-- To make debugging such issues easier, we still log.
+			log("debug", "replacing incompatible existing metric family %s", name)
+			-- Below is the code to be strict.
+			--error("conflicting declarations for metric family "..name)
+		else
+			return existing
+		end
+	end
+	self.families[name] = metric_family
+	return metric_family
+end
+
+function metric_registry_mt:gauge(name, unit, description, labels, extra)
+	name = compose_name(name, unit)
+	local mf = new_metric_family(self.backend, "gauge", name, unit, description, labels, extra)
+	mf = self:register_metric_family(name, mf)
+	return mf
+end
+
+function metric_registry_mt:counter(name, unit, description, labels, extra)
+	name = compose_name(name, unit)
+	local mf = new_metric_family(self.backend, "counter", name, unit, description, labels, extra)
+	mf = self:register_metric_family(name, mf)
+	return mf
+end
+
+function metric_registry_mt:histogram(name, unit, description, labels, extra)
+	name = compose_name(name, unit)
+	local mf = new_metric_family(self.backend, "histogram", name, unit, description, labels, extra)
+	mf = self:register_metric_family(name, mf)
+	return mf
+end
+
+function metric_registry_mt:summary(name, unit, description, labels, extra)
+	name = compose_name(name, unit)
+	local mf = new_metric_family(self.backend, "summary", name, unit, description, labels, extra)
+	mf = self:register_metric_family(name, mf)
+	return mf
+end
+
+function metric_registry_mt:get_metric_families()
+	return self.families
+end
+
+-- END of MetricRegistry implementation
+
+-- BEGIN of general helpers for implementing high-level APIs on top of OpenMetrics
+
+local function timed(metric)
+	local t0 = time()
+	local submitter = assert(metric.sample or metric.set, "metric type cannot be used with timed()")
+	return function()
+		local t1 = time()
+		submitter(metric, t1-t0)
+	end
+end
+
+-- END of general helpers
+
+return {
+	new_metric_proxy = new_metric_proxy;
+	new_metric_registry = new_metric_registry;
+	render_histogram_le = render_histogram_le;
+	timed = timed;
+}
diff --git a/util/statistics.lua b/util/statistics.lua
index db608217..a8401168 100644
--- a/util/statistics.lua
+++ b/util/statistics.lua
@@ -1,171 +1,191 @@
-local t_sort = table.sort
-local m_floor = math.floor;
 local time = require "util.time".now;
+local new_metric_registry = require "util.openmetrics".new_metric_registry;
+local render_histogram_le = require "util.openmetrics".render_histogram_le;
 
-local function nop_function() end
+-- BEGIN of Metric implementations
 
-local function percentile(arr, length, pc)
-	local n = pc/100 * (length + 1);
-	local k, d = m_floor(n), n%1;
-	if k == 0 then
-		return arr[1] or 0;
-	elseif k >= length then
-		return arr[length];
-	end
-	return arr[k] + d*(arr[k+1] - arr[k]);
+-- Gauges
+local gauge_metric_mt = {}
+gauge_metric_mt.__index = gauge_metric_mt
+
+local function new_gauge_metric()
+	local metric = { value = 0 }
+	setmetatable(metric, gauge_metric_mt)
+	return metric
+end
+
+function gauge_metric_mt:set(value)
+	self.value = value
+end
+
+function gauge_metric_mt:add(delta)
+	self.value = self.value + delta
 end
 
-local function new_registry(config)
-	config = config or {};
-	local duration_sample_interval = config.duration_sample_interval or 5;
-	local duration_max_samples = config.duration_max_stored_samples or 5000;
+function gauge_metric_mt:reset()
+	self.value = 0
+end
 
-	local function get_distribution_stats(events, n_actual_events, since, new_time, units)
-		local n_stored_events = #events;
-		t_sort(events);
-		local sum = 0;
-		for i = 1, n_stored_events do
-			sum = sum + events[i];
+function gauge_metric_mt:iter_samples()
+	local done = false
+	return function(_s)
+		if done then
+			return nil, true
 		end
+		done = true
+		return "", nil, _s.value
+	end, self
+end
 
-		return {
-			samples = events;
-			sample_count = n_stored_events;
-			count = n_actual_events,
-			rate = n_actual_events/(new_time-since);
-			average = n_stored_events > 0 and sum/n_stored_events or 0,
-			min = events[1] or 0,
-			max = events[n_stored_events] or 0,
-			units = units,
-		};
-	end
+-- Counters
+local counter_metric_mt = {}
+counter_metric_mt.__index = counter_metric_mt
+
+local function new_counter_metric()
+	local metric = {
+		_created = time(),
+		value = 0,
+	}
+	setmetatable(metric, counter_metric_mt)
+	return metric
+end
 
+function counter_metric_mt:set(value)
+	self.value = value
+end
 
-	local registry = {};
-	local methods;
-	methods = {
-		amount = function (name, conf)
-			local v = conf and conf.initial or 0;
-			registry[name..":amount"] = function ()
-				return "amount", v, conf;
-			end
-			return function (new_v) v = new_v; end
-		end;
-		counter = function (name, conf)
-			local v = conf and conf.initial or 0;
-			registry[name..":amount"] = function ()
-				return "amount", v, conf;
-			end
-			return function (delta)
-				v = v + delta;
-			end;
-		end;
-		rate = function (name, conf)
-			local since, n, total = time(), 0, 0;
-			registry[name..":rate"] = function ()
-				total = total + n;
-				local t = time();
-				local stats = {
-					rate = n/(t-since);
-					count = n;
-					total = total;
-					units = conf and conf.units;
-					type = conf and conf.type;
-				};
-				since, n = t, 0;
-				return "rate", stats.rate, stats;
-			end;
-			return function ()
-				n = n + 1;
-			end;
-		end;
-		distribution = function (name, conf)
-			local units = conf and conf.units;
-			local type = conf and conf.type or "distribution";
-			local events, last_event = {}, 0;
-			local n_actual_events = 0;
-			local since = time();
-
-			registry[name..":"..type] = function ()
-				local new_time = time();
-				local stats = get_distribution_stats(events, n_actual_events, since, new_time, units);
-				events, last_event = {}, 0;
-				n_actual_events = 0;
-				since = new_time;
-				return type, stats.average, stats;
-			end;
-
-			return function (value)
-				n_actual_events = n_actual_events + 1;
-				if n_actual_events%duration_sample_interval == 1 then
-					last_event = (last_event%duration_max_samples) + 1;
-					events[last_event] = value;
-				end
-			end;
-		end;
-		sizes = function (name, conf)
-			conf = conf or { units = "bytes", type = "size" }
-			return methods.distribution(name, conf);
-		end;
-		times = function (name, conf)
-			local units = conf and conf.units or "seconds";
-			local events, last_event = {}, 0;
-			local n_actual_events = 0;
-			local since = time();
-
-			registry[name..":duration"] = function ()
-				local new_time = time();
-				local stats = get_distribution_stats(events, n_actual_events, since, new_time, units);
-				events, last_event = {}, 0;
-				n_actual_events = 0;
-				since = new_time;
-				return "duration", stats.average, stats;
-			end;
-
-			return function ()
-				n_actual_events = n_actual_events + 1;
-				if n_actual_events%duration_sample_interval ~= 1 then
-					return nop_function;
-				end
-
-				local start_time = time();
-				return function ()
-					local end_time = time();
-					local duration = end_time - start_time;
-					last_event = (last_event%duration_max_samples) + 1;
-					events[last_event] = duration;
-				end
-			end;
-		end;
-
-		get_stats = function ()
-			return registry;
-		end;
-	};
-	return methods;
+function counter_metric_mt:add(value)
+	self.value = (self.value or 0) + value
 end
 
-return {
-	new = new_registry;
-	get_histogram = function (duration, n_buckets)
-		n_buckets = n_buckets or 100;
-		local events, n_events = duration.samples, duration.sample_count;
-		if not (events and n_events) then
-			return nil, "not a valid distribution stat";
+function counter_metric_mt:iter_samples()
+	local step = 0
+	return function(_s)
+		step = step + 1
+		if step == 1 then
+			return "_created", nil, _s._created
+		elseif step == 2 then
+			return "_total", nil, _s.value
+		else
+			return nil, nil, true
+		end
+	end, self
+end
+
+function counter_metric_mt:reset()
+	self.value = 0
+end
+
+-- Histograms
+local histogram_metric_mt = {}
+histogram_metric_mt.__index = histogram_metric_mt
+
+local function new_histogram_metric(buckets)
+	local metric = {
+		_created = time(),
+		_sum = 0,
+		_count = 0,
+	}
+	-- the order of buckets matters unfortunately, so we cannot directly use
+	-- the thresholds as table keys
+	for i, threshold in ipairs(buckets) do
+		metric[i] = {
+			threshold = threshold,
+			threshold_s = render_histogram_le(threshold),
+			count = 0
+		}
+	end
+	setmetatable(metric, histogram_metric_mt)
+	return metric
+end
+
+function histogram_metric_mt:sample(value)
+	-- According to the I-D, values must be part of all buckets
+	for i, bucket in pairs(self) do
+		if "number" == type(i) and bucket.threshold > value then
+			bucket.count = bucket.count + 1
 		end
-		local histogram = {};
+	end
+	self._sum = self._sum + value
+	self._count = self._count + 1
+end
 
-		for i = 1, 100, 100/n_buckets do
-			histogram[i] = percentile(events, n_events, i);
+function histogram_metric_mt:iter_samples()
+	local key = nil
+	return function (_s)
+		local data
+		key, data = next(_s, key)
+		if key == "_created" or key == "_sum" or key == "_count" then
+			return key, nil, data
+		elseif key ~= nil then
+			return "_bucket", {["le"] = data.threshold_s}, data.count
+		else
+			return nil, nil, nil
 		end
-		return histogram;
-	end;
+	end, self
+end
 
-	get_percentile = function (duration, pc)
-		local events, n_events = duration.samples, duration.sample_count;
-		if not (events and n_events) then
-			return nil, "not a valid distribution stat";
+function histogram_metric_mt:reset()
+	self._created = time()
+	self._count = 0
+	self._sum = 0
+	for i, bucket in pairs(self) do
+		if "number" == type(i) then
+			bucket.count = 0
 		end
-		return percentile(events, n_events, pc);
-	end;
+	end
+end
+
+-- Summary
+local summary_metric_mt = {}
+summary_metric_mt.__index = summary_metric_mt
+
+local function new_summary_metric()
+	-- quantiles are not supported yet
+	local metric = {
+		_created = time(),
+		_sum = 0,
+		_count = 0,
+	}
+	setmetatable(metric, summary_metric_mt)
+	return metric
+end
+
+function summary_metric_mt:sample(value)
+	self._sum = self._sum + value
+	self._count = self._count + 1
+end
+
+function summary_metric_mt:iter_samples()
+	local key = nil
+	return function (_s)
+		local data
+		key, data = next(_s, key)
+		return key, nil, data
+	end, self
+end
+
+function summary_metric_mt:reset()
+	self._created = time()
+	self._count = 0
+	self._sum = 0
+end
+
+local pull_backend = {
+	gauge = new_gauge_metric,
+	counter = new_counter_metric,
+	histogram = new_histogram_metric,
+	summary = new_summary_metric,
+}
+
+-- END of Metric implementations
+
+local function new()
+	return {
+		metric_registry = new_metric_registry(pull_backend),
+	}
+end
+
+return {
+	new = new;
 }
diff --git a/util/statsd.lua b/util/statsd.lua
index 8f6151c6..25e03e38 100644
--- a/util/statsd.lua
+++ b/util/statsd.lua
@@ -1,82 +1,267 @@
 local socket = require "socket";
+local time = require "util.time".now;
+local array = require "util.array";
+local t_concat = table.concat;
 
-local time = require "util.time".now
+local new_metric_registry = require "util.openmetrics".new_metric_registry;
+local render_histogram_le = require "util.openmetrics".render_histogram_le;
 
-local function new(config)
-	if not config or not config.statsd_server then
-		return nil, "No statsd server specified in the config, please see https://prosody.im/doc/statistics";
+-- BEGIN of Metric implementations
+
+-- Gauges
+local gauge_metric_mt = {}
+gauge_metric_mt.__index = gauge_metric_mt
+
+local function new_gauge_metric(full_name, impl)
+	local metric = {
+		_full_name = full_name;
+		_impl = impl;
+		value = 0;
+	}
+	setmetatable(metric, gauge_metric_mt)
+	return metric
+end
+
+function gauge_metric_mt:set(value)
+	self.value = value
+	self._impl:push_gauge(self._full_name, value)
+end
+
+function gauge_metric_mt:add(delta)
+	self.value = self.value + delta
+	self._impl:push_gauge(self._full_name, self.value)
+end
+
+function gauge_metric_mt:reset()
+	self.value = 0
+	self._impl:push_gauge(self._full_name, 0)
+end
+
+function gauge_metric_mt.iter_samples()
+	-- statsd backend does not support iteration.
+	return function()
+		return nil
 	end
+end
 
-	local sock = socket.udp();
-	sock:setpeername(config.statsd_server, config.statsd_port or 8125);
+-- Counters
+local counter_metric_mt = {}
+counter_metric_mt.__index = counter_metric_mt
 
-	local prefix = (config.prefix or "prosody")..".";
+local function new_counter_metric(full_name, impl)
+	local metric = {
+		_full_name = full_name,
+		_impl = impl,
+		value = 0,
+	}
+	setmetatable(metric, counter_metric_mt)
+	return metric
+end
+
+function counter_metric_mt:set(value)
+	local delta = value - self.value
+	self.value = value
+	self._impl:push_counter_delta(self._full_name, delta)
+end
 
-	local function send_metric(s)
-		return sock:send(prefix..s);
+function counter_metric_mt:add(value)
+	self.value = (self.value or 0) + value
+	self._impl:push_counter_delta(self._full_name, value)
+end
+
+function counter_metric_mt.iter_samples()
+	-- statsd backend does not support iteration.
+	return function()
+		return nil
+	end
+end
+
+function counter_metric_mt:reset()
+	self.value = 0
+end
+
+-- Histograms
+local histogram_metric_mt = {}
+histogram_metric_mt.__index = histogram_metric_mt
+
+local function new_histogram_metric(buckets, full_name, impl)
+	-- NOTE: even though the more or less proprietrary dogstatsd has its own
+	-- histogram implementation, we push the individual buckets in this statsd
+	-- backend for both consistency and compatibility across statsd
+	-- implementations.
+	local metric = {
+		_sum_name = full_name..".sum",
+		_count_name = full_name..".count",
+		_impl = impl,
+		_created = time(),
+		_sum = 0,
+		_count = 0,
+	}
+	-- the order of buckets matters unfortunately, so we cannot directly use
+	-- the thresholds as table keys
+	for i, threshold in ipairs(buckets) do
+		local threshold_s = render_histogram_le(threshold)
+		metric[i] = {
+			threshold = threshold,
+			threshold_s = threshold_s,
+			count = 0,
+			_full_name = full_name..".bucket."..(threshold_s:gsub("%.", "_")),
+		}
 	end
+	setmetatable(metric, histogram_metric_mt)
+	return metric
+end
 
-	local function send_gauge(name, amount, relative)
-		local s_amount = tostring(amount);
-		if relative and amount > 0 then
-			s_amount = "+"..s_amount;
+function histogram_metric_mt:sample(value)
+	-- According to the I-D, values must be part of all buckets
+	for i, bucket in pairs(self) do
+		if "number" == type(i) and bucket.threshold > value then
+			bucket.count = bucket.count + 1
+			self._impl:push_counter_delta(bucket._full_name, 1)
 		end
-		return send_metric(name..":"..s_amount.."|g");
 	end
+	self._sum = self._sum + value
+	self._count = self._count + 1
+	self._impl:push_gauge(self._sum_name, self._sum)
+	self._impl:push_counter_delta(self._count_name, 1)
+end
 
-	local function send_counter(name, amount)
-		return send_metric(name..":"..tostring(amount).."|c");
+function histogram_metric_mt.iter_samples()
+	-- statsd backend does not support iteration.
+	return function()
+		return nil
 	end
+end
 
-	local function send_duration(name, duration)
-		return send_metric(name..":"..tostring(duration).."|ms");
+function histogram_metric_mt:reset()
+	self._created = time()
+	self._count = 0
+	self._sum = 0
+	for i, bucket in pairs(self) do
+		if "number" == type(i) then
+			bucket.count = 0
+		end
 	end
+	self._impl:push_gauge(self._sum_name, self._sum)
+end
+
+-- Summaries
+local summary_metric_mt = {}
+summary_metric_mt.__index = summary_metric_mt
+
+local function new_summary_metric(full_name, impl)
+	local metric = {
+		_sum_name = full_name..".sum",
+		_count_name = full_name..".count",
+		_impl = impl,
+	}
+	setmetatable(metric, summary_metric_mt)
+	return metric
+end
+
+function summary_metric_mt:sample(value)
+	self._impl:push_counter_delta(self._sum_name, value)
+	self._impl:push_counter_delta(self._count_name, 1)
+end
 
-	local function send_histogram_sample(name, sample)
-		return send_metric(name..":"..tostring(sample).."|h");
+function summary_metric_mt.iter_samples()
+	-- statsd backend does not support iteration.
+	return function()
+		return nil
 	end
+end
 
-	local methods;
-	methods = {
-		amount = function (name, conf)
-			if conf and conf.initial then
-				send_gauge(name, conf.initial);
-			end
-			return function (new_v) send_gauge(name, new_v); end
-		end;
-		counter = function (name, conf) --luacheck: ignore 212/conf
-			return function (delta)
-				send_gauge(name, delta, true);
-			end;
-		end;
-		rate = function (name)
-			return function ()
-				send_counter(name, 1);
-			end;
+function summary_metric_mt.reset()
+end
+
+-- BEGIN of statsd client implementation
+
+local statsd_mt = {}
+statsd_mt.__index = statsd_mt
+
+function statsd_mt:cork()
+	self.corked = true
+	self.cork_buffer = self.cork_buffer or {}
+end
+
+function statsd_mt:uncork()
+	self.corked = false
+	self:_flush_cork_buffer()
+end
+
+function statsd_mt:_flush_cork_buffer()
+	local buffer = self.cork_buffer
+	for metric_name, value in pairs(buffer) do
+		self:_send_gauge(metric_name, value)
+		buffer[metric_name] = nil
+	end
+end
+
+function statsd_mt:push_gauge(metric_name, value)
+	if self.corked then
+		self.cork_buffer[metric_name] = value
+	else
+		self:_send_gauge(metric_name, value)
+	end
+end
+
+function statsd_mt:_send_gauge(metric_name, value)
+	self:_send(self.prefix..metric_name..":"..tostring(value).."|g")
+end
+
+function statsd_mt:push_counter_delta(metric_name, delta)
+	self:_send(self.prefix..metric_name..":"..tostring(delta).."|c")
+end
+
+function statsd_mt:_send(s)
+	return self.sock:send(s)
+end
+
+-- END of statsd client implementation
+
+local function build_metric_name(family_name, labels)
+	local parts = array { family_name }
+	if labels then
+		parts:append(labels)
+	end
+	return t_concat(parts, "/"):gsub("%.", "_"):gsub("/", ".")
+end
+
+local function new(config)
+	if not config or not config.statsd_server then
+		return nil, "No statsd server specified in the config, please see https://prosody.im/doc/statistics";
+	end
+
+	local sock = socket.udp();
+	sock:setpeername(config.statsd_server, config.statsd_port or 8125);
+
+	local prefix = (config.prefix or "prosody")..".";
+
+	local impl = {
+		metric_registry = nil;
+		sock = sock;
+		prefix = prefix;
+	};
+	setmetatable(impl, statsd_mt)
+
+	local backend = {
+		gauge = function(family_name, labels)
+			return new_gauge_metric(build_metric_name(family_name, labels), impl)
 		end;
-		distribution = function (name, conf) --luacheck: ignore 212/conf
-			return function (value)
-				send_histogram_sample(name, value);
-			end;
+		counter = function(family_name, labels)
+			return new_counter_metric(build_metric_name(family_name, labels), impl)
 		end;
-		sizes = function (name)
-			name = name.."_size";
-			return function (value)
-				send_histogram_sample(name, value);
-			end;
+		histogram = function(buckets, family_name, labels)
+			return new_histogram_metric(buckets, build_metric_name(family_name, labels), impl)
 		end;
-		times = function (name)
-			return function ()
-				local start_time = time();
-				return function ()
-					local end_time = time();
-					local duration = end_time - start_time;
-					send_duration(name, duration*1000);
-				end
-			end;
+		summary = function(family_name, labels, extra)
+			return new_summary_metric(build_metric_name(family_name, labels), impl, extra)
 		end;
 	};
-	return methods;
+
+	impl.metric_registry = new_metric_registry(backend);
+
+	return impl;
 end
 
 return {