HADOOP-9291. enhance unit-test coverage of package o.a.h.metrics2 (Ivan A. Veselovsky via jeagles)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1534476 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/branch-2' into branch-2-gd
Merge remote branch 'origin/branch-2-gd' into fb-YARN-474-branch-2-gd