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/notifier.go

@@ -31,6 +31,7 @@ func newNotifier(b *backend) (eventNotifier, error) {
 			tracer:          b.tracer,
 			bulkLock:        b.bulkLock,
 			listLatestRVs:   b.listLatestRVs,
+			storageMetrics:  b.storageMetrics,
 			historyPoll: func(ctx context.Context, grp string, res string, since int64) ([]*historyPollResponse, error) {
 				var records []*historyPollResponse
 				err := b.db.WithTx(ctx, ReadCommittedRO, func(ctx context.Context, tx db.Tx) error {