svn merge -c 1476487 FIXES: HADOOP-9504. MetricsDynamicMBeanBase has concurrency issues in createMBeanInfo. Contributed by Liang Xie


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-0.23@1476489 13f79535-47bb-0310-9956-ffa450edef68