s2s et al.: Add counters for connection state transitions

mod_s2s: Port to new OpenMetrics API

mod_c2s: Port to new OpenMetrics API

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

util.prosodyctl.shell: Fix for different location of unpack in Lua 5.1

util.prosodyctl.shell: Fix for missing 'sep' arg to string.rep in Lua 5.1

util.prosodyctl.shell: Allow calling console commands with fewer shell quotes E.g. `prosodyctl shell module reload disco example.com` becomes equivalent to `prosodyctl shell 'module:reload("disco", "example.com")`. Won't work for every possible command, but reduces the amount of shell quoting problems for most common commands.

mod_tls: Bail out if session got destroyed while sending <proceed/> Can happen in case opportunistic_writes is enabled and the session got destroyed while writing that tag. Thanks Ge0rG

core.sessionmanager: Add a dummy rawsend() method to resting sessions Should fix a traceback on attempted use after destruction, in case where opportunistic_writes was in use. Thanks Ge0rG

mod_c2s: Fix traceback if session was destroyed while opening stream (thanks Ge0rG) Could happen with the 'opportunistic_writes' setting, since then the stream opening is written directly to the socket, which can in turn trigger session destruction if the socket somehow got closed just after the other sent their stream header. Error happens later when it tries to `hosts[session.host == nil].events`