Add OWNERS for metrics.

It looks like all the other people who made significant contributions
to this project have left the team.

BUG=none
TEST=none

Change-Id: Ie572a43e85cee565c71c4101d1474e77e51449e5
Reviewed-on: https://chromium-review.googlesource.com/192008


Reviewed-by: Luigi Semenzato <semenzato@chromium.org>
Tested-by: Luigi Semenzato <semenzato@chromium.org>
Commit-Queue: Luigi Semenzato <semenzato@chromium.org>