* more than 2 per second:
 *
 * {@snippet :
 * final RateLimiter rateLimiter = RateLimiter.create(2.0); // rate is "2 permits per second"
 * void submitTasks(List<Runnable> tasks, Executor executor) {
 *   for (Runnable task : tasks) {
 *     rateLimiter.acquire(); // may wait
 *     executor.execute(task);
 *   }
 * }
 * }
 *