metrics: make meter updates lock-free (#21446)

metrics/meter_test.go

@@ -17,7 +17,7 @@ func TestGetOrRegisterMeter(t *testing.T) {
 	r := NewRegistry()
 	NewRegisteredMeter("foo", r).Mark(47)
 	if m := GetOrRegisterMeter("foo", r); m.Count() != 47 {
-		t.Fatal(m)
+		t.Fatal(m.Count())
 	}
 }
 
@@ -29,10 +29,11 @@ func TestMeterDecay(t *testing.T) {
 	defer ma.ticker.Stop()
 	m := newStandardMeter()
 	ma.meters[m] = struct{}{}
-	go ma.tick()
 	m.Mark(1)
+	ma.tickMeters()
 	rateMean := m.RateMean()
 	time.Sleep(100 * time.Millisecond)
+	ma.tickMeters()
 	if m.RateMean() >= rateMean {
 		t.Error("m.RateMean() didn't decrease")
 	}