*       <blockquote>
   *       maxPermits = thresholdPermits + 2 * warmupPeriod / (stableInterval + coldInterval)
   *       </blockquote>
   * </ul>
   */
  static final class SmoothWarmingUp extends SmoothRateLimiter {
    private final long warmupPeriodMicros;

    /**
     * The slope of the line from the stable interval (when permits == 0), to the cold interval
     * (when permits == maxPermits)