This is where it was originally implemented:

https://codereview.chromium.org/633373011

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/b9c826014f8b9b270cdf5792086bb265c31fbf1a

commit b9c826014f8b9b270cdf5792086bb265c31fbf1a
Author: Alexei Svitkine <asvitkine@chromium.org>
Date: Sat Oct 13 00:19:26 2018

Fix UMA/UKM ActualUploadInterval metrics.

This was likely broken by refactoring for UKM.

Previously, last_upload_finish_time_ was never set and
thus the metrics weren't being logged.

Bug:  894909 
Change-Id: Idd0472b2f17bf85a4997fa6003893b6ed1d901e8
Reviewed-on: https://chromium-review.googlesource.com/c/1278226
Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599437}
[modify] https://crrev.com/b9c826014f8b9b270cdf5792086bb265c31fbf1a/components/metrics/reporting_service.cc
[modify] https://crrev.com/b9c826014f8b9b270cdf5792086bb265c31fbf1a/tools/metrics/histograms/histograms.xml

Only seeing values of 0 currently:

https://uma.googleplex.com/p/chrome/histograms/?endDate=20181020&dayCount=7&histograms=UMA.ActualLogUploadInterval&fixupData=true&showMax=true&filters=platform%2Ceq%2CM%2Cchannel%2Ceq%2C1%2Cmilestone%2Ceq%2C72%2Cisofficial%2Ceq%2CTrue&implicitFilters=isofficial

So seems there's still an issue.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/01f7205e196ef47e8aa11e93d2b62687fc6977f2

commit 01f7205e196ef47e8aa11e93d2b62687fc6977f2
Author: Alexei Svitkine <asvitkine@chromium.org>
Date: Thu Nov 01 23:45:04 2018

Fix UMA/UKM ActualLogUploadInterval metrics some more.

https://chromium-review.googlesource.com/c/chromium/src/+/1278226
attempted to fix these, but there was still an issue because
last_upload_finish_time_ was being set to TimeTicks() instead of
Now().

Bug:  894909 
Change-Id: Ibacc8755e53ed6a67fa0ba5a52cea1f281f32c36
Reviewed-on: https://chromium-review.googlesource.com/c/1310639
Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604756}
[modify] https://crrev.com/01f7205e196ef47e8aa11e93d2b62687fc6977f2/components/metrics/reporting_service.cc

The last change landed in 72.0.3599.0.

Looks fixed:

https://uma.googleplex.com/p/chrome/histograms/?endDate=20181102&dayCount=1&histograms=UMA.ActualLogUploadInterval&fixupData=true&showMax=true&filters=platform%2Ceq%2CM%2Cchannel%2Ceq%2C1%2Csimple_version%2Ceq%2C72.0.3599.0%2Cisofficial%2Ceq%2CTrue&implicitFilters=isofficial