LU-12585 mdt: Add read/write latency to MDT stats

LU-12585 mdt: Add read/write latency to MDT stats

The MDT does not currently record latency stats for reads
and writes.

Add this, and change the naming to be the same as for the
OFD.

Note on this:
Existing naming on the MDT uses "read/write" instead of
"{read,write}_bytes", which is inconsistent with OFD and
also inconsistent within the MDT, since other ops without
the "_bytes" suffix are latency.

It's not ideal to change the names of existing stats, but I
decided this was less problematic than leaving them
inconsistent.

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I7b7a5742678cbe0269086f37877833e877a5ca5f
Reviewed-on: https://review.whamcloud.com/46229
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Aurelien Degremont <degremoa@amazon.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>