* @return time in microseconds to wait until the resource can be acquired, never negative
   */
  final long reserve(int permits) {
    checkPermits(permits);
    synchronized (mutex()) {
      return reserveAndGetWaitLength(permits, stopwatch.readMicros());
    }
  }

  /**
   * Acquires a permit from this {@code RateLimiter} if it can be obtained without exceeding the