fix (unified-storage): stop registering unified storage metrics in global state (#101322)

* move prometheus.register for unified storage metrics into metrics.go and do most of the plumbing to get it to work

* convert StorageApiMetrics to pointer and check for nil before using it

* rename type and variables to something more sensible

---------

Co-authored-by: Jean-Philippe Quéméner <jeanphilippe.quemener@grafana.com>

pkg/tests/testinfra/testinfra.go

@@ -104,7 +104,7 @@ func StartGrafanaEnv(t *testing.T, grafDir, cfgPath string) (string, *server.Tes
 	var storage sql.UnifiedStorageGrpcService
 	if runstore {
 		storage, err = sql.ProvideUnifiedStorageGrpcService(env.Cfg, env.FeatureToggles, env.SQLStore,
-			env.Cfg.Logger, prometheus.NewPedanticRegistry(), nil)
+			env.Cfg.Logger, prometheus.NewPedanticRegistry(), nil, nil)
 		require.NoError(t, err)
 		ctx := context.Background()
 		err = storage.StartAsync(ctx)