Ability to get delay time from RateLimiter

[cowwoc]

I am implementing a rate-limited non-blocking thread pool. Upon hitting the rate limit, I would like a timer to wake up threads when the next permit is expected to expire. Please provide a method that returns the minimum amount of time the caller would have to wait before the next permit can be acquired.

[Stephan202]

This sounds like a generalization of #3044 (return a long rather than a boolean).

[mkobit]

Something along these lines would be useful to enable simpler usage of RateLimiter from Kotlin coroutines. The delay time could be use for something like suspend fun delay since RateLimiter implementation seems to currently use com.google.common.util.concurrent.Uninterruptibles#sleepUninterruptibly.