Since metrics are related, one good place to instrument such UMA metric could be https://cs.chromium.org/chromium/src/components/sync/engine_impl/syncer_proto_util.cc?l=336&rcl=dd6d6cf97b6a95c258cb7deb49b6a3ead32ddad1

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/cc6c745c687e366e6da39a6152d0d9e2c38469ad

commit cc6c745c687e366e6da39a6152d0d9e2c38469ad
Author: Mikel Astiz <mastiz@chromium.org>
Date: Mon Jan 14 21:54:04 2019

Add UMA for errors returned by sync server

This should allow investigate future issues and perhaps even do some
basic monitoring, for scenarios like users being throttled.

Bug: 921026,921588
Change-Id: Iff6036cd43863450f7561075cd8f9efb5a3a9f70
Reviewed-on: https://chromium-review.googlesource.com/c/1407084
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622620}
[modify] https://crrev.com/cc6c745c687e366e6da39a6152d0d9e2c38469ad/components/sync/engine_impl/syncer_proto_util.cc
[modify] https://crrev.com/cc6c745c687e366e6da39a6152d0d9e2c38469ad/tools/metrics/histograms/enums.xml
[modify] https://crrev.com/cc6c745c687e366e6da39a6152d0d9e2c38469ad/tools/metrics/histograms/histograms.xml

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/f638a1ea2d17c61303bf46706f3f5a1926ffa6b4

commit f638a1ea2d17c61303bf46706f3f5a1926ffa6b4
Author: Mikel Astiz <mastiz@chromium.org>
Date: Mon Jan 14 22:18:42 2019

Add UMA for sync server round trip latency

This should allow investigate future issues and also better understand
the headroom for improving latency of certain features.

Bug: 921026,902343
Change-Id: I05366ce4c08f102f47965bcbd4beb67d28352642
Reviewed-on: https://chromium-review.googlesource.com/c/1407085
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622628}
[modify] https://crrev.com/f638a1ea2d17c61303bf46706f3f5a1926ffa6b4/components/sync/engine_impl/syncer_proto_util.cc
[modify] https://crrev.com/f638a1ea2d17c61303bf46706f3f5a1926ffa6b4/tools/metrics/histograms/histograms.xml

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d18eac6936e3c36c7b93c3a4811205879545c3f6

commit d18eac6936e3c36c7b93c3a4811205879545c3f6
Author: Mikel Astiz <mastiz@chromium.org>
Date: Tue Jan 15 07:50:40 2019

Add UMA to break down sync GetUpdates() per datatype

This metric should allow investigating regressions like increased
traffic load to sync servers.

Bug: 921026,921588
Change-Id: I93668afceab67014e43af01c24c2342120ab23ea
Reviewed-on: https://chromium-review.googlesource.com/c/1409555
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Jan Krcal <jkrcal@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622771}
[modify] https://crrev.com/d18eac6936e3c36c7b93c3a4811205879545c3f6/chrome/browser/sync/test/integration/enable_disable_test.cc
[modify] https://crrev.com/d18eac6936e3c36c7b93c3a4811205879545c3f6/components/sync/engine_impl/syncer_proto_util.cc
[modify] https://crrev.com/d18eac6936e3c36c7b93c3a4811205879545c3f6/tools/metrics/histograms/histograms.xml

Requesting merge for the three patches above, which add a new UMA metric each. These metrics are very relevant for rollouts and experiments in M72, including https://bugs.chromium.org/p/chromium/issues/approval?id=902704

All three patches are pretty simple and very unlikely to cause issues. The first two have been verified on latest canary.

This bug requires manual review: We are only 13 days from stable.
Please contact the milestone owner if you have questions.
Owners: govind@(Android), kariahda@(iOS), djmm@(ChromeOS), abdulsyed@(Desktop)

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot