platform2: Add libmetrics slotted library targets.

BUG=chromium:342866
CQ-DEPEND=CL:185187
CQ-DEPEND=CL:186026
TEST=Trybot run on paladin, release, and chromiumos-sdk builders.

Change-Id: I6d6d2c16c91640774988995aecc93226eda3d353
Reviewed-on: https://chromium-review.googlesource.com/186037
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>
Commit-Queue: Ben Chan <benchan@chromium.org>
1 file changed