fix race in testutil Accumulator.Wait() (#2598)

testutil/accumulator.go

@@ -43,9 +43,9 @@ func (a *Accumulator) NMetrics() uint64 {
 }
 
 func (a *Accumulator) ClearMetrics() {
-	atomic.StoreUint64(&a.nMetrics, 0)
 	a.Lock()
 	defer a.Unlock()
+	atomic.StoreUint64(&a.nMetrics, 0)
 	a.Metrics = make([]*Metric, 0)
 }
 
@@ -56,9 +56,9 @@ func (a *Accumulator) AddFields(
 	tags map[string]string,
 	timestamp ...time.Time,
 ) {
-	atomic.AddUint64(&a.nMetrics, 1)
 	a.Lock()
 	defer a.Unlock()
+	atomic.AddUint64(&a.nMetrics, 1)
 	if a.Cond != nil {
 		a.Cond.Broadcast()
 	}