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We strongly discourage histograms that have an expiry date more than 1 year out.

Also, add a link to the design doc for automatic expiry date management for
histograms that we can detect as actively in use.

R=bcwhite@chromium.org

Bug: none
Change-Id: Ifcfcd831c80e047799be49d6c14d21ba4bb4e49f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1935728
Commit-Queue: Ilya Sherman <isherman@chromium.org>
Auto-Submit: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Cr-Commit-Position: refs/heads/master@{#719455}