Partial workaround for timer during a clock jump

`time.Now()` uses the wall-clock and therefore calling it twice, one
second apart, can produce quite different results (negative results but
also a difference greater than one second). Go doesn't expose a
monotonic clock, therefore, it's difficult to properly fix (but there is
https://github.com/gavv/monotime if we want a proper fix for
`timer.Time()`).

This workaround just discard any non-positive duration.

timer.go

@@ -215,18 +215,18 @@ func (t *StandardTimer) Time(f func()) {
 
 // Record the duration of an event.
 func (t *StandardTimer) Update(d time.Duration) {
-	t.mutex.Lock()
-	defer t.mutex.Unlock()
-	t.histogram.Update(int64(d))
-	t.meter.Mark(1)
+	// Partial workaround for detecting a clock change
+	if d >= 0 {
+		t.mutex.Lock()
+		defer t.mutex.Unlock()
+		t.histogram.Update(int64(d))
+		t.meter.Mark(1)
+	}
 }
 
 // Record the duration of an event that started at a time and ends now.
 func (t *StandardTimer) UpdateSince(ts time.Time) {
-	t.mutex.Lock()
-	defer t.mutex.Unlock()
-	t.histogram.Update(int64(time.Since(ts)))
-	t.meter.Mark(1)
+	t.Update(time.Since(ts))
 }
 
 // Variance returns the variance of the values in the sample.

timer_test.go

@@ -27,6 +27,7 @@ func TestTimerExtremes(t *testing.T) {
 	tm := NewTimer()
 	tm.Update(math.MaxInt64)
 	tm.Update(0)
+	tm.Update(math.MinInt64)
 	if stdDev := tm.StdDev(); 4.611686018427388e+18 != stdDev {
 		t.Errorf("tm.StdDev(): 4.611686018427388e+18 != %v\n", stdDev)
 	}