code: func (l *TimeLimiter) add(now time.Time, d time.Duration) {
  	if now.After(l.next) {
  		l.timeSpent = d
  		l.since = now.Add(-d)
  		l.next = l.since.Add(l.window)
  		return
  	}
  	l.timeSpent += d
  }
docs: // add increments the "time spent" counter by "d" at a given time