code: func (t *Timer) ObserveDuration() time.Duration {
  	d := time.Since(t.begin)
  	if t.observer != nil {
  		t.observer.Observe(d.Seconds())
  	}
  	return d
  }
docs: // ObserveDuration records the duration passed since the Timer was created with
  // NewTimer. It calls the Observe method of the Observer provided during
  // construction with the duration in seconds as an argument. The observed
  // duration is also returned. ObserveDuration is usually called with a defer
  // statement.
  //
  // Note that this method is only guaranteed to never observe negative durations
  // if used with Go1.9+.