code: func (r *EwmaRate) Update(now time.Time) float64 {
  	timeDelta := now.Sub(r.lastTimestamp)
  	return r.Ewma.Update(nanosec/float64(timeDelta.Nanoseconds()), now)
  }
docs: // Notify of an event happening, with specified current time.
  //
  // Returns current rate.