RegionMetricsStorage could be a better name this class holds and mutates the numbers that will be exposed as dynamic region metrics.
I'll add the comments, and check the line length. Not sure how I messed up eclipse.
The data members can be package private now that they are in the regionserver.metrics package. I'll check to make sure.
Everything else is accessed in the regionserver package so they need to be public :-/
TestRegionServerMetrics covers most of the functionality of the new class but I can create a new set of more explicit tests if you think that is needed.