Refine the code.
Merge branch 'hbase-11339-metrics' of https://github.com/JingchengDu/hbase into hbase-11339-metrics
Merge branch 'hbase-11339-metrics' of https://github.com/JingchengDu/hbase into hbase-11339-metrics
Add the metrics for mob compactor.
Add ut for the metrics.
Refine the code.