metrics: fix -Winconsistent-missing-override warning

BUG=chromium:453069
TEST=FEATURES="test" emerge-amd64-generic metrics with new clang.

Change-Id: I1576200c94a01af46dd2f36e87806b8c93de7b86
Reviewed-on: https://chromium-review.googlesource.com/244052


Reviewed-by: Dan Erat <derat@chromium.org>
Commit-Queue: Yunlian Jiang <yunlian@chromium.org>
Tested-by: Yunlian Jiang <yunlian@chromium.org>