HBASE-10561 Forward port: HBASE-10212 New rpc metric: number of active handler

git-svn-id: https://svn.apache.org/repos/asf/hbase/trunk@1594117 13f79535-47bb-0310-9956-ffa450edef68
Amend HBASE-10561 Forward port: HBASE-10212 New rpc metric: number of active handler

git-svn-id: https://svn.apache.org/repos/asf/hbase/trunk@1594126 13f79535-47bb-0310-9956-ffa450edef68