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>

cmd/root.go

@@ -10,6 +10,7 @@ import (
 
 	"github.com/navidrome/navidrome/conf"
 	"github.com/navidrome/navidrome/consts"
+	"github.com/navidrome/navidrome/core"
 	"github.com/navidrome/navidrome/db"
 	"github.com/navidrome/navidrome/log"
 	"github.com/navidrome/navidrome/resources"
@@ -91,6 +92,8 @@ func startServer(ctx context.Context) func() error {
 			a.MountRouter("ListenBrainz Auth", consts.URLPathNativeAPI+"/listenbrainz", CreateListenBrainzRouter())
 		}
 		if conf.Server.Prometheus.Enabled {
+			// blocking call because takes <1ms but useful if fails
+			core.WriteInitialMetrics()
 			a.MountRouter("Prometheus metrics", conf.Server.Prometheus.MetricsPath, promhttp.Handler())
 		}
 		if strings.HasPrefix(conf.Server.UILoginBackgroundURL, "/") {