feat(openmetrics): Add basic call metrics

[SystemKeeper]

Add some basic talk metrics to the OpenMetrics endpoint:

# TYPE nextcloud_talk_active_calls gauge
# UNIT nextcloud_talk_active_calls calls
# HELP nextcloud_talk_active_calls Number of active calls in talk
nextcloud_talk_active_calls 0 1769185753

# TYPE nextcloud_talk_participants_in_calls gauge
# UNIT nextcloud_talk_participants_in_calls calas
# HELP nextcloud_talk_participants_in_calls Number of participants in calls in talk
nextcloud_talk_participants_in_calls 0 1769185753

• Needs fix(openmetrics): Multiple exporters per app server#57765
• CI red until appinfo.xml schema does not allow openmetrics element server#57766

🏁 Checklist

• ⛑️ Tests (unit and/or integration) are included or not possible
• 📘 API documentation in docs/ has been updated or is not required
• 🔖 Capability is added or not needed

[SystemKeeper]

• appinfo.xml schema does not allow openmetrics element server#57766

[nickvergessen]

Nitpick: I'd call it sessions, as many users with the same user id count as multiple and cause load like multiple, but it might be a confusing detail.

The unit however has to change to sessions or participants

[SystemKeeper]

I would then also change participants -> sessions in

spreed/lib/Command/Monitor/HasActiveCalls.php

Lines 47 to 54 in 0a496b8

	$numParticipants = $this->metricsService->getNumberOfParticipantsInCalls();
	if ($input->getOption('output') === 'plain') {
	$output->writeln(sprintf('<error>There are currently %1$d calls in progress with %2$d participants</error>', $numCalls, $numParticipants));
	} else {
	$data = ['calls' => $numCalls, 'participants' => $numParticipants];
	$this->writeArrayInOutputFormat($input, $output, $data);
	}

, which would be breaking for non-plain output. Fine or should we keep participants here (or additionally as fallback)?

[nickvergessen]

I'd not change the command to not break scripting done with it

[SystemKeeper]

Unit specifies MetricFamily units. If non-empty, it MUST be a suffix of the MetricFamily name separated by an underscore. Be aware that further generation rules might make it an infix in the text format.

https://prometheus.io/docs/specs/om/open_metrics_spec/#unit

Needs to be fixed. Server has currently the same issue, it's only a SHOULD in version 2.0.0, but we export 1.0.0 and there it's a MUST.

[SystemKeeper]

Fixed in last commit

[nickvergessen]

/backport to stable33