Add metrics GYP file

BUG=chromium:220003
TEST=Run emerge-<board> -v platform2 with and without platform2 USE
flag on a range of boards plus the host

Change-Id: I44f15c31a8ac3917f3262912c43419a93652c0d8
Reviewed-on: https://gerrit.chromium.org/gerrit/40316


Tested-by: Liam McLoughlin <lmcloughlin@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Liam McLoughlin <lmcloughlin@chromium.org>