Spoke to Metrics about this. They said it's fine to send as a UMA histogram.

We have two options for how we do this:

1. Log it on some particular event eg browser startup

- Downside: biased towards users who do more of that event, eg. start up Chrome more often. Showing unique users helps, but still won't include data from users who didn't open Chrome in the time period of interest.

2. Log it with every metrics upload

- Upside: least-biased, as will include data from users that sent *any* data in the time period of interest, regardless of whether they started the browser 

- Upside: can be used as a filter for other metrics (?somehow?: TODO:followup)

- Downside: Won't show up on Finch dashboards (but will on uma histogram / timeline dashboards)


I think we should go with (2) as this will give us the 'truest' answer to the question of 'what % of chrome users have disabled chrome notifications' and allow more powerful querying down the line.

To do (2), we need to log our new histrogram in the 'ProvideCurrentSessionData' method of a MetricsProvider. There is an existing metrics provider, AndroidMetricsProvider, that we can hook into to do this.

https://cs.chromium.org/chromium/src/chrome/browser/metrics/android_metrics_provider.cc

Just to clarify, the Finch just doesn't support seeing unique user counts for a histogram. (Doesn't have the selector at the top right like histograms and timeline dash.)

So that's a downside for any time you want to count users for a metric, not for this specific one.

Otherwise, everything above is accurate.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/03f68475750d964f87fa3ffa20f6018859fe9e19

commit 03f68475750d964f87fa3ffa20f6018859fe9e19
Author: Anita Woodruff <awdf@chromium.org>
Date: Tue Jul 31 14:06:35 2018

[Android Metrics] Add tests for AndroidMetricsProvider

- Just testing the bucket counts for now because it would be non-trivial
to introduce seams to allow manipulation of the values to be logged
from the unit test.

- This sets me up nicely to add a new test for the new metric I am adding
as part of  https://crbug.com/786017  .

Bug:  786017 
Change-Id: Ic9c538e71f42a12decc2046b8323bc1cb19e81cc
Reviewed-on: https://chromium-review.googlesource.com/1154540
Commit-Queue: Anita Woodruff <awdf@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579393}
[add] https://crrev.com/03f68475750d964f87fa3ffa20f6018859fe9e19/chrome/browser/metrics/android_metrics_provider_unittest.cc
[modify] https://crrev.com/03f68475750d964f87fa3ffa20f6018859fe9e19/chrome/test/BUILD.gn

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/af99e6efb10ece96ab258804a41fb01669ad9c66

commit af99e6efb10ece96ab258804a41fb01669ad9c66
Author: Anita Woodruff <awdf@chromium.org>
Date: Tue Jul 31 15:00:23 2018

[Android Metrics] Always record app notification status

- Previously we only recorded the app notification status when a
notification was sent.

- Now we record whether notifications are enabled on every metrics
upload. This will allow us to easily answer the question 'what
percentage of users currently have notifications disabled' regardless
of whether they received any notifications since the last upload.

- Keeping the old histograms for now since they are still useful for
seeing what percentage of notifications are blocked.

Bug:  786017 
Change-Id: I1926650836a9b85edc9e38d1bb852217949f47fa
Reviewed-on: https://chromium-review.googlesource.com/1155112
Commit-Queue: Anita Woodruff <awdf@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579408}
[modify] https://crrev.com/af99e6efb10ece96ab258804a41fb01669ad9c66/chrome/android/java/src/org/chromium/chrome/browser/notifications/NotificationPlatformBridge.java
[modify] https://crrev.com/af99e6efb10ece96ab258804a41fb01669ad9c66/chrome/android/java/src/org/chromium/chrome/browser/notifications/NotificationSystemStatusUtil.java
[modify] https://crrev.com/af99e6efb10ece96ab258804a41fb01669ad9c66/chrome/android/java_sources.gni
[add] https://crrev.com/af99e6efb10ece96ab258804a41fb01669ad9c66/chrome/android/junit/src/org/chromium/chrome/browser/notifications/NotificationSystemStatusUtilUnitTest.java
[modify] https://crrev.com/af99e6efb10ece96ab258804a41fb01669ad9c66/chrome/browser/BUILD.gn
[modify] https://crrev.com/af99e6efb10ece96ab258804a41fb01669ad9c66/chrome/browser/metrics/android_metrics_provider.cc
[modify] https://crrev.com/af99e6efb10ece96ab258804a41fb01669ad9c66/chrome/browser/metrics/android_metrics_provider_unittest.cc
[modify] https://crrev.com/af99e6efb10ece96ab258804a41fb01669ad9c66/tools/metrics/histograms/histograms.xml

Resolving as fixed - this should get logged in the Android.AppNotificationStatus histogram from now on