feat(openmetrics): introduce OpenMetrics exporter

[Altahrim]

Summary

Introduce OpenMetrics endpoint featuring Nextcloud metrics.

It allows to get statistics about the Nextcloud instance but not about hardware (some exporters are already doing that very well). For example: number of files, shares or active users.

It's extensible so every app will be able to expose it's own metrics.

Test

Access it on /metrics (or index.php/metrics depending of configuration)

TODO

• OpenAPI extractor

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (see feat: add documentation for OpenMetrics documentation#13947)
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

[icewind1991]

Some of these (counting files) are going to have a non-trivial cost, might be worth having a way to toggle individual metrics.

[Altahrim]

Some of these (counting files) are going to have a non-trivial cost, might be worth having a way to toggle individual metrics.

You are right :) That's why I added a way to disable a metric with openmetrics_skipped_classes and also the Cached exporter. I hope both mechanisms will be enough (maybe cached exporter will need to be configurable to run only once per hour or even less)

[icewind1991]

Some of these (counting files) are going to have a non-trivial cost, might be worth having a way to toggle individual metrics.

You are right :) That's why I added a way to disable a metric with openmetrics_skipped_classes and also the Cached exporter. I hope both mechanisms will be enough (maybe cached exporter will need to be configurable to run only once per hour or even less)

Ok, missed that when skimming over the changes 🙈

[tcitworld]

This should be OCP\IDBConnection

[tcitworld]

Not LoggerInterface ?

[Altahrim]

You are right for both, thank you. I'll do a follow-up PR :)

[rizlas]

Hi,
sorry for bother, but am I wrong or haven't any metrics regarding groups been implemented?

[Altahrim]

@rizlas You are right, there isn't any for now but the exporter can be extended.
What kind of metric do you have in mind?

[susnux]

New public API needs mention here:
https://github.com/nextcloud/documentation/blob/1a0415c8bf1541e90ae0b4da487a362a51a1cfe2/developer_manual/app_publishing_maintenance/app_upgrade_guide/upgrade_to_33.rst?plain=1#L184

[rizlas]

@Altahrim Thanks! I was basically thinking about the number of groups and users per group.
It would be nice to also extend https://github.com/nextcloud/groupquota (as specified here https://docs.nextcloud.com/server/latest/developer_manual//digging_deeper/openmetrics.html#implement-a-new-exporter) with: quota per group, quota used per group and free quota per group.

[Altahrim]

@rizlas Number of groups and number of users per group can be added in Nextcloud, but I am afraid we don't have any straightforward way to get the metrics.
For the number of users per group, I don't know what would be the best statistic. Maybe min/max/avg size or a histogram…

All metrics related to quota must be added in groupquota app as you mentioned :)