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/manager_test.go

@@ -27,7 +27,7 @@ var _ = Describe("Plugin Manager", func() {
 		conf.Server.Plugins.Folder = testDataDir
 
 		ctx = GinkgoT().Context()
-		mgr = createManager(nil)
+		mgr = createManager(nil, nil)
 		mgr.ScanPlugins()
 	})
 
@@ -85,7 +85,7 @@ var _ = Describe("Plugin Manager", func() {
 			})
 
 			conf.Server.Plugins.Folder = tempPluginsDir
-			m = createManager(nil)
+			m = createManager(nil, nil)
 		})
 
 		// Helper to create a complete valid plugin for manager testing