On busy spurts we can see regionservers start to see large queues for compaction. It's really hard to tell if the server is queueing a lot of compactions for the same region, lots of compactions for lots of regions, or just falling behind.
For flushes much the same. There can be flushes in queue that aren't being run because of delayed flushes. There's no way to know from the metrics how many flushes are for each region, how many are delayed. Etc.
We should add either more metrics around this ( num per region, max per region, min per region ) or add on a UI page that has the list of compactions and flushes.