libmetrics: add pkg-config file

BUG=chromium:366984
TEST=`FEATURES=test emerge-$BOARD platform2` and verify libmetrics-242728.pc is created.

Change-Id: I3cd79e7421a5f666a4bf2551c87086f779078771
Reviewed-on: https://chromium-review.googlesource.com/196974


Reviewed-by: Mike Frysinger <vapier@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>
Commit-Queue: Ben Chan <benchan@chromium.org>