Merge -c 1491989 from trunk to branch-2.1-beta to fix YARN-767. Initialize application metrics at RM bootup. Contributed by Jian He.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2.1-beta@1491991 13f79535-47bb-0310-9956-ffa450edef68