ceph: add a new metric to keep track of remote object copies

This patch adds latency and size metrics for remote object copies
operations ("copyfrom").  For now, these metrics will be available on the
client only, they won't be sent to the MDS.

Signed-off-by: Luís Henriques <lhenriques@suse.de>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>