code: func (r *Number) Sum(now time.Time) float64 {
  	sum := float64(0)
  
  	r.Mutex.RLock()
  	defer r.Mutex.RUnlock()
  
  	for timestamp, bucket := range r.Buckets {
  		// TODO: configurable rolling window
  		if timestamp >= now.Unix()-10 {
  			sum += bucket.Value
  		}
  	}
  
  	return sum
  }
docs: // Sum sums the values over the buckets in the last 10 seconds.