FR: Add metrics to the prometheus exporter

[ShadwDrgn]

Not sure if this is possible for a plugin but some prometheus metrics around playback would be awesome.

[DuvelCorp]

Please do it!

[ShadwDrgn]

The idea here is that we could get metrics such as number of watch minutes seconds or hours or w/e with labels around which episode or movie it was what series, which user it was and which device, etc. then we could design our own dashboards around most watched content or w/e we want.