[rpc] add metric for AcceptorPool's dispatch timing

This patch adds 'acceptor_dispatch_times' histogram metric to track
dispatching times of newly accepted connections by AcceptorPool along
with a very basic unit test for the newly added metric.

Change-Id: I018ddd14414c8d13aaf488fa9eb4db1bf1248cc4
Reviewed-on: http://gerrit.cloudera.org:8080/20790
Tested-by: Alexey Serbin <alexey@apache.org>
Reviewed-by: Abhishek Chennaka <achennaka@cloudera.com>
3 files changed