The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/da716cb0d544f3c5ca778458b16702a2659a5012

commit da716cb0d544f3c5ca778458b16702a2659a5012
Author: Joshua Pawlicki <waffles@chromium.org>
Date: Wed Jun 27 15:32:04 2018

Add UMA to record update client error codes.

Bug:  855724 
Change-Id: Ib0a7ccc2f2fe272d53d19ef833b47dcf3994b0ba
Reviewed-on: https://chromium-review.googlesource.com/1114096
Reviewed-by: Sorin Jianu <sorin@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Joshua Pawlicki <waffles@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570783}
[modify] https://crrev.com/da716cb0d544f3c5ca778458b16702a2659a5012/components/component_updater/component_updater_service.cc
[modify] https://crrev.com/da716cb0d544f3c5ca778458b16702a2659a5012/components/update_client/update_client_errors.h
[modify] https://crrev.com/da716cb0d544f3c5ca778458b16702a2659a5012/tools/metrics/histograms/enums.xml
[modify] https://crrev.com/da716cb0d544f3c5ca778458b16702a2659a5012/tools/metrics/histograms/histograms.xml

This is due to an increased number of RETRY_LATERS, triggered by a server-side release on May 17th or 18th.

Before: We would issue Retry-After (10 hours) to any request that doesn't have an update-check from 0.0.0.0.

After: We issue a Retry-After (10 hours) to any request that is background priority.

I suspect the M67 rollout is when we added the explicit "bg" header. In M66, we would have fallen back to our fg/bg heuristics, which includes the update-check-from-0.0.0.0 as sufficient to promote the traffic to fg, which is why for M66 the old is equivalent to the new.

I believe we are issuing this header by mistake anyways, and will drop it from the config within the next hour (after I confirm expected QPS impact).