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/storage/unified/sql/test/integration_test.go

@@ -83,7 +83,7 @@ func TestClientServer(t *testing.T) {
 
 	features := featuremgmt.WithFeatures()
 
-	svc, err := sql.ProvideUnifiedStorageGrpcService(cfg, features, dbstore, nil, prometheus.NewPedanticRegistry(), nil)
+	svc, err := sql.ProvideUnifiedStorageGrpcService(cfg, features, dbstore, nil, prometheus.NewPedanticRegistry(), nil, nil)
 	require.NoError(t, err)
 	var client resource.ResourceStoreClient