client/rpc: Add ping and msgs stats

This can be seen as below,

># cat $META/graphs/active/vol-client-0/private |grep ping_msgs_sent
ping_msgs_sent = 2

># cat $META/graphs/active/vol-client-0/private |grep "^msgs_sent"
msgs_sent = 13

where $META is /<fuse-mountpt>/.meta

Change-Id: I2107ec2b045bac701377760635e18758adb943a3
Signed-off-by: Krishnan Parthasarathi <kparthas@redhat.com>
Reviewed-on: http://review.gluster.org/8285
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Atin Mukherjee <amukherj@redhat.com>
Reviewed-by: Anand Avati <avati@redhat.com>