ceph: include average/stdev r/w/m latency in mds metrics

stdev is computed in `cephfs-top` tool - clients forward
square of sums and IO count required to calculate stdev.

Signed-off-by: Venky Shankar <vshankar@redhat.com>
Reviewed-by: Xiubo Li <xiubli@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>