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

1 files changed, 173 insertions, 153 deletions

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;
 }