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, 26 insertions, 19 deletions

diff --git a/core/moduleapi.lua b/core/moduleapi.lua
index 82002737..59417027 100644
--- a/core/moduleapi.lua
+++ b/core/moduleapi.lua
@@ -510,26 +510,33 @@ end
 
 function api:measure(name, stat_type, conf)
 	local measure = require "core.statsmanager".measure;
-	return measure(stat_type, "/"..self.host.."/mod_"..self.name.."/"..name, conf);
-end
-
-function api:measure_object_event(events_object, event_name, stat_name)
-	local m = self:measure(stat_name or event_name, "times");
-	local function handler(handlers, _event_name, _event_data)
-		local finished = m();
-		local ret = handlers(_event_name, _event_data);
-		finished();
-		return ret;
+	local fixed_label_key, fixed_label_value
+	if self.host ~= "*" then
+		fixed_label_key = "host"
+		fixed_label_value = self.host
 	end
-	return self:hook_object_event(events_object, event_name, handler);
-end
-
-function api:measure_event(event_name, stat_name)
-	return self:measure_object_event((hosts[self.host] or prosody).events.wrappers, event_name, stat_name);
-end
-
-function api:measure_global_event(event_name, stat_name)
-	return self:measure_object_event(prosody.events.wrappers, event_name, stat_name);
+	-- new_legacy_metric takes care of scoping for us, as it does not accept
+	-- an array of labels
+	-- the prosody_ prefix is automatically added by statsmanager for legacy
+	-- metrics.
+	return measure(stat_type, "mod_"..self.name.."/"..name, conf, fixed_label_key, fixed_label_value)
+end
+
+function api:metric(type_, name, unit, description, label_keys, conf)
+	local metric = require "core.statsmanager".metric;
+	local is_scoped = self.host ~= "*"
+	if is_scoped then
+		-- prepend `host` label to label keys if this is not a global module
+		local orig_labels = label_keys
+		label_keys = array { "host" }
+		label_keys:append(orig_labels)
+	end
+	local mf = metric(type_, "prosody_mod_"..self.name.."/"..name, unit, description, label_keys, conf)
+	if is_scoped then
+		-- make sure to scope the returned metric family to the current host
+		return mf:with_partial_label(self.host)
+	end
+	return mf
 end
 
 local status_priorities = { error = 3, warn = 2, info = 1, core = 0 };