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/server/wire.go

@@ -400,6 +400,8 @@ var wireBasicSet = wire.NewSet(
 	connectors.ProvideOrgRoleMapper,
 	wire.Bind(new(user.Verifier), new(*userimpl.Verifier)),
 	authz.WireSet,
+	// Unified storage
+	resource.ProvideStorageMetrics,
 	// Kubernetes API server
 	grafanaapiserver.WireSet,
 	apiregistry.WireSet,