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/cmd/grafana-cli/commands/datamigrations/to_unified_storage.go

@@ -196,7 +196,7 @@ func newUnifiedClient(cfg *setting.Cfg, sqlStore db.DB) (resource.ResourceClient
 		Reg:      prometheus.NewPedanticRegistry(),
 		Authzc:   authlib.FixedAccessClient(true), // always true!
 		Docs:     nil,                             // document supplier
-	})
+	}, nil)
 }
 
 func newParquetClient(file *os.File) (resource.BulkStoreClient, error) {