Base SQL metrics in MetricsWorker (#2002)

* feat: Add metrics worker

* refactor: Add todos for useful for metrics methods

* feat: Run MetricsWorker is Prometheus is Enabled

* refactor: Unused low-level variable was removed in metrics

* feat: No worker for metrics, add more

* refactor: Unnecessary todo removed

* refactor: Remove dead unused constant

* Reduce metrics public interface

Co-authored-by: Deluan <deluan@navidrome.org>

model/player.go

@@ -24,4 +24,5 @@ type PlayerRepository interface {
 	Get(id string) (*Player, error)
 	FindMatch(userName, client, typ string) (*Player, error)
 	Put(p *Player) error
+	// TODO: Add CountAll method. Useful at least for metrics.
 }