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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

-- Storage Interface API Description
--
-- This is written as a TypedLua description

-- Key-Value stores (the default)

interface keyval_store
	get : ( self, string? ) -> (any) | (nil, string)
	set : ( self, string?, any ) -> (boolean) | (nil, string)
end

-- Map stores (key-key-value stores)

interface map_store
	get : ( self, string?, any ) -> (any) | (nil, string)
	set : ( self, string?, any, any ) -> (boolean) | (nil, string)
	set_keys : ( self, string?, { any : any }) -> (boolean) | (nil, string)
	remove : {}
end

-- Archive stores

typealias archive_query = {
	"start"  : number?, -- timestamp
	"end"    : number?, -- timestamp
	"with"   : string?,
	"after"  : string?, -- archive id
	"before" : string?, -- archive id
	"total"  : boolean?,
}

interface archive_store
	-- Optional set of capabilities
	caps   : {
		-- Optional total count of matching items returned as second return value from :find()
		"total" : boolean?,
	}?

	-- Add to the archive
	append : ( self, string?, string?, any, number?, string? ) -> (string) | (nil, string)

	-- Iterate over archive
	find   : ( self, string?, archive_query? ) -> ( () -> ( string, any, number?, string? ), integer? )

	-- Removal of items. API like find. Optional?
	delete : ( self, string?, archive_query? ) -> (boolean) | (number) | (nil, string)

	-- Array of dates which do have messages (Optional?)
	dates  : ( self, string? ) -> ({ string }) | (nil, string)

	-- Map of counts per "with" field
	summary : ( self, string?, archive_query? ) -> ( { string : integer } ) | (nil, string)

	-- Map-store API
	get    : ( self, string, string ) -> (stanza, number?, string?) | (nil, string)
	set    : ( self, string, string, stanza, number?, string? ) -> (boolean) | (nil, string)
end

-- This represents moduleapi
interface module
	-- If the first string is omitted then the name of the module is used
	-- The second string is one of "keyval" (default), "map" or "archive"
	open_store : (self, string?, string?) -> (keyval_store) | (map_store) | (archive_store) | (nil, string)

	-- Other module methods omitted
end

module : module