mv non-core components to extended repo

pkg/metrics/nop.go

@@ -0,0 +1,23 @@
+package metrics
+
+var (
+	nilGauge    = &nopGauge{}
+	nilCounter  = &nopCounter{}
+	nilObserver = &nopObserver{}
+)
+
+type nopGauge struct{}
+
+func (*nopGauge) Inc()          {}
+func (*nopGauge) Dec()          {}
+func (*nopGauge) Add(v float64) {}
+func (*nopGauge) Set(v float64) {}
+
+type nopCounter struct{}
+
+func (*nopCounter) Inc()          {}
+func (*nopCounter) Add(v float64) {}
+
+type nopObserver struct{}
+
+func (*nopObserver) Observe(v float64) {}