Make it easier to configure rate limiter to never cross max_burst admitted cells in a specific period of time

[kskalski]

I spent some time debugging why rate limiter allowed total n larger than max_burst within first minute when using Quota::per_minute(max_burst).

It seems this is actually an allowed and correct behavior according to the algorithm and careful reading of documentation. What happened was basically: some of the cost cells got replenished before client requested admitting the subsequent max_burst + Xth cell.

Maybe the documentation could be still improved, e.g. stating explicitly that above scenario is possible, but I'm wondering about the best way to configure rate limiter for a use case where the "moving window" admittance of cells in a period specified when creating quota is always below max_burst.
My current simple approach is "down-scaling" the period, i.e. something like this:

 // `governor` counts cost accumulated *minus* replenished in time, so it won't
 // strictly forbid going over max burst in specified period of time (during
 // steady ramp-up initial requests will be forgotten before full period).
 // Downscaling quota to smaller granularity period mostly prevents this problem.
 let divisor = 10.min((minute_limit + rate_limits::MAX_COST - 1) / rate_limits::MAX_COST);
 let max_period_burst =
     rate_limits::Cost::new(minute_limit / divisor).unwrap_or(rate_limits::Cost::MIN);
 let unit_replienish_in = std::time::Duration::from_millis(60000 / minute_limit as u64);
 governor::Quota::with_period(unit_replienish_in).unwrap().allow_burst(max_period_burst)

One idea that would make it easier is a version of above code to be added to quota, so I could do something like
governor::Quota::per_minute(minute_limit).with_granularity(Duration::from_secs(10))