Base Prometheus metrics handler

[AlekseyLobanov]

I want to add some standard metrics to navidrome. Prometheus metrics are useful for observability (including alerts).
I believe that many simple PRs are much better than 1 full.

What this PR has:

1. Two options for enable/disable (default for compatibility) metrics and their path
2. Standard prometheus handler with default golang metrics like process_start_time_seconds, go_threads and process_cpu_seconds_total
3. Does not consume resources if disabled.

I believe that metrics path is best approach, compare to different port or some auth, including HTTP Basic Auth:

1. If navidrome works behind reverse proxy, closing /metrics is the most simple way to protect them
2. Otherwise, metrics path like /metrics_super_secret_path is simple to use and secure enough. The most serious problem in this case is plain HTTP connection over not secure connection which is not secure anyway.

This relates to #1548 , but not solves initial request for now

[deluan]

Hey, sorry for the long delay! LGTM, just need to rebase with latest from master and I'll merge it right away. Thanks!

[AlekseyLobanov]

Branch is now actual

[deluan]

Thanks!

[anarcat]

hi! are there any plans to expand this to (say) count the number of users, albums, artists, tracks, etc?

[djjudas21]

Thanks for the addition. How is this feature enabled with env vars? I need to add support for metrics to the helm chart but I'm not sure how the Go options correspond to env vars. Is prometheus.enabled equivalent to ND_PROMETHEUS_ENABLED?

[AlekseyLobanov]

How is this feature enabled with env vars? I need to add support for metrics to the helm chart but I'm not sure how the Go options correspond to env vars. Is prometheus.enabled equivalent to ND_PROMETHEUS_ENABLED?

I should add this to docs repository, definitely.

To enable metrics:

1. Define variable ND_PROMETHEUS_ENABLED=true
2. (optional) Use custom metrics path ND_PROMETHEUS_METRICSPATH=/metrics_secret_key

UPD: PR with documentation updates: navidrome/website#96

[AlekseyLobanov]

hi! are there any plans to expand this to (say) count the number of users, albums, artists, tracks, etc?

Of course!

It was my first PR to this project (and first PR to golang project), I tried to create it as small as possible. It has only required dependencies and configuration layer required by Prometheus to run.

Next step is adding navidrome-specfic metrics. I have plans to do this in near future.

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