feat(prometheus): add metrics to Subsonic API and Plugins (#4266)

* Add prometheus metrics to subsonic and plugins

* address feedback, do not log error if operation is not supported

* add missing timestamp and client to stats

* remove .view from subsonic route

* directly inject DataStore in Prometheus, to avoid having to pass it in every call

Signed-off-by: Deluan <deluan@navidrome.org>

---------

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

plugins/host_scheduler_test.go

@@ -16,7 +16,7 @@ var _ = Describe("SchedulerService", func() {
 	)
 
 	BeforeEach(func() {
-		manager = createManager(nil)
+		manager = createManager(nil, nil)
 		ss = manager.schedulerService
 	})