HBASE-8397 improve unit-test coverage of package org.apache.hadoop.hbase.master.metrics (0.94)



git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.94@1537607 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/0.94' into 0.94-gd
Merge remote branch 'origin/0.94-gd' into fb-HBASE-8559-0.94-gd