[Feature Request] Add OpenMetrics/Prometheus endpoint to expose YS internal metrics

[Byh0ki]

Describe the issue

We currently don't have a way to follow the data that is ingested/collected by YS. It would be nice to expose some internal metrics on a web server to allow metrics collection by Prometheus for example.

Expected behavior

Expose the metrics on a separated web server/port:

• number of ingested songs/artists/album
• number of users / status of each last sync for each users
• number of songs listened per day
• etc