Do not register compat metrics in amtool (#3713)

There is no need to register these metrics in amtool, so use
compat.NewMetrics(nil) instead of compat.RegisteredMetrics.

Signed-off-by: George Robinson <george.robinson@grafana.com>

cli/root.go

@@ -61,7 +61,7 @@ func initMatchersCompat(_ *kingpin.ParseContext) error {
 	if err != nil {
 		kingpin.Fatalf("error parsing the feature flag list: %v\n", err)
 	}
-	compat.InitFromFlags(logger, compat.RegisteredMetrics, featureConfig)
+	compat.InitFromFlags(logger, compat.NewMetrics(nil), featureConfig)
 	return nil
 }