jobserver: enable prometheus metrics when running standalone jobserver (#18636)

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

cmd/mattermost/commands/init.go

@@ -44,6 +44,7 @@ func initDBCommandContext(configDSN string, readOnlyConfigStore bool) (*app.App,
 	s, err := app.NewServer(
 		app.Config(configDSN, readOnlyConfigStore, nil),
 		app.StartSearchEngine,
+		app.StartMetrics,
 	)
 	if err != nil {
 		return nil, err