Add anonymous namespace to components/metrics tests.

go/c-readability-advice#anonymous-namespaces-in-tests

Change-Id: I84398d82693e6fe33d1f303053d4c7280e9a0e9a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7011793
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Ramon Cano Aparicio <rcanoaparicio@google.com>
Cr-Commit-Position: refs/heads/main@{#1526287}
NOKEYCHECK=True
GitOrigin-RevId: bc6fa1b780538927d65d0cd93edb0ae1015ca845