meter: fixed another race reported by the race-detector

Author: James DeFelice

meter.go

@@ -147,10 +147,7 @@ func newStandardMeter() *StandardMeter {
 
 // Stop stops the meter, Mark() will be a no-op if you use it after being stopped.
 func (m *StandardMeter) Stop() {
-	m.lock.Lock()
-	stopped := m.stopped
-	m.stopped = 1
-	m.lock.Unlock()
+	stopped := atomic.SwapUint32(&m.stopped, 1)
 	if stopped != 1 {
 		arbiter.Lock()
 		defer arbiter.Unlock()
@@ -247,7 +244,7 @@ type meterArbiter struct {
 	started bool
 	meters  map[*StandardMeter]struct{}
 	ticker  *time.Ticker
-	cancel chan struct{}
+	cancel  chan struct{}
 }
 
 var arbiter = meterArbiter{ticker: time.NewTicker(5e9), meters: make(map[*StandardMeter]struct{})}