HADOOP-9504. MetricsDynamicMBeanBase has concurrency issues in createMBeanInfo. Contributed by Liang Xie


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1476487 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/trunk' into trunk-gd
Merge remote branch 'origin/trunk-gd' into fb-HADOOP-7435-TEST-trunk-gd