HADOOP-11163. MetricsSystemImpl may miss a registered source. Contributed by Chuan Liu.

(cherry picked from commit 69c9af91031f345d9511618877646b8756b17863)
HADOOP-11163. Correct CHANGES.txt.

(cherry picked from commit bbb3b1acc0e8ce10b8689ded2be73b0965f09e52)