edr = ExponentiallyDecayingReservoir.

* * Record the accurate number of events within each given time interval, one for each bucket.

* this implementation dependent on the MetricRegistry.

* * An exponentially-decaying random reservoir of longs.

* * The default implementation of GetFastCompass * All the data will be saved in memory.

usually, this struct is used to support test.

A random sampling reservoir of a stream of int64s.