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>

scanner/scanner.go

@@ -151,8 +151,10 @@ func (s *scanner) RescanAll(ctx context.Context, fullRescan bool) error {
 	}
 	if hasError {
 		log.Error("Errors while scanning media. Please check the logs")
+		core.WriteAfterScanMetrics(ctx, s.ds, false)
 		return ErrScanError
 	}
+	core.WriteAfterScanMetrics(ctx, s.ds, true)
 	return nil
 }