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The metric isn't used frequently, but when needed it can help see
trends in incoming traffic metrics.

BUG=975882

Change-Id: Ic3e1e2a3b3ad273a4d8d128f82068142f7315950
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1713231Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Ted Choc <tedchoc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#717722}