YARN-2157. Added YARN metrics in the documentaion. Contributed by Akira AJISAKA
(cherry picked from commit 90a968d6757511b6d89538516db0e699129d854c)
YARN-2870. Updated the command to run the timeline server it the document. Contributed by Masatake Iwasaki.

(cherry picked from commit ef38fb9758f230c3021e70b749d7a11f8bac03f5)