swarm/shed: add metrics to each shed db (#18277)

* swarm/shed: add metrics to each shed db

* swarm/shed: push metrics prefix up

* swarm/shed: rename prefix to metricsPrefix

* swarm/shed: unexport Meter, remove Mutex for quit channel

swarm/shed/example_store_test.go

@@ -52,7 +52,7 @@ type Store struct {
 // and possible conflicts with schema from existing database is checked
 // automatically.
 func New(path string) (s *Store, err error) {
-	db, err := shed.NewDB(path)
+	db, err := shed.NewDB(path, "")
 	if err != nil {
 		return nil, err
 	}