ceph: add IO size metrics support

ceph: add IO size metrics support

This will collect IO's total size and then calculate the average
size, and also will collect the min/max IO sizes.

The debugfs will show the size metrics in bytes and will let the
userspace applications to switch to what they need.

URL: https://tracker.ceph.com/issues/49913
Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>