HADOOP-9661. Allow metrics sources to be extended. (sandyr via tucu)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1496228 13f79535-47bb-0310-9956-ffa450edef68
HADOOP-9355.  Abstract symlink tests to use either FileContext or FileSystem.  (Andrew Wang via Colin Patrick McCabe)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1496306 13f79535-47bb-0310-9956-ffa450edef68