metrics: Fix the import path of protobufs.

We are switching chromeos-base/metrics to use the metrics component
pulled as part of the checkout.
This components will be copied in $S and not in platform2/metrics. The
import path need to be fixed.

BUG=chromium:474811
TEST=trybots.
TEST=unittests.

CQ-DEPEND=CL:264544

Change-Id: I02a379eff0f97081dffd382bbee5028b3600f975
Reviewed-on: https://chromium-review.googlesource.com/264506


Tested-by: Bertrand Simonnet <bsimonnet@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Bertrand Simonnet <bsimonnet@chromium.org>