metrics: fix unittest

This fixes a compilation bug that was not caught because of crbug.com/392504.

BUG=chromium:392504
TEST=FEATURES=test emerge-amd64-generic metrics

Change-Id: Id71a8945d1ec80c7e5d7f2d8de5d287321b47de7
Reviewed-on: https://chromium-review.googlesource.com/207131


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