metrics: zero temp variable in updateMeter (#21470)

* metrics: zero temp variable in  updateMeter

Previously the temp variable was not updated properly after summing it to count.
This meant we had astronomically high metrics, now we zero out the temp whenever we
sum it onto the snapshot count

* metrics: move temp variable to be aligned, unit tests

Moves the temp variable in MeterSnapshot to be 64-bit aligned because of the atomic bug.
Adds a unit test, that catches the previous bug.

metrics/meter.go

@@ -101,8 +101,12 @@ func NewRegisteredMeterForced(name string, r Registry) Meter {
 
 // MeterSnapshot is a read-only copy of another Meter.
 type MeterSnapshot struct {
-	count                          int64
+	// WARNING: The `temp` field is accessed atomically.
+	// On 32 bit platforms, only 64-bit aligned fields can be atomic. The struct is
+	// guaranteed to be so aligned, so take advantage of that. For more information,
+	// see https://golang.org/pkg/sync/atomic/#pkg-note-BUG.
 	temp                           int64
+	count                          int64
 	rate1, rate5, rate15, rateMean float64
 }
 
@@ -253,7 +257,7 @@ func (m *StandardMeter) updateSnapshot() {
 
 func (m *StandardMeter) updateMeter() {
 	// should only run with write lock held on m.lock
-	n := atomic.LoadInt64(&m.snapshot.temp)
+	n := atomic.SwapInt64(&m.snapshot.temp, 0)
 	m.snapshot.count += n
 	m.a1.Update(n)
 	m.a5.Update(n)

metrics/meter_test.go

@@ -73,3 +73,19 @@ func TestMeterZero(t *testing.T) {
 		t.Errorf("m.Count(): 0 != %v\n", count)
 	}
 }
+
+func TestMeterRepeat(t *testing.T) {
+	m := NewMeter()
+	for i := 0; i < 101; i++ {
+		m.Mark(int64(i))
+	}
+	if count := m.Count(); count != 5050 {
+		t.Errorf("m.Count(): 5050 != %v\n", count)
+	}
+	for i := 0; i < 101; i++ {
+		m.Mark(int64(i))
+	}
+	if count := m.Count(); count != 10100 {
+		t.Errorf("m.Count(): 10100 != %v\n", count)
+	}
+}