metrics: Use the new serialization mechanism in libmetrics

Convert libmetrics to the new serialization mechanism living in
//components/metrics in the chrome repo.

BUG=chromium:374368
TEST=FEATURES=test emerge-amd64-generic metrics

Change-Id: Ia8cf128d04fedd9672fb47096dc6fd87d6a9043d
Reviewed-on: https://chromium-review.googlesource.com/207237


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