I think this is fine, but what we really need in order to determine where folks drop off is a notion of 'last action clicked'.  I.e., there are multiple paths to connecting, and we should have an enumeration of which UI elements were interacted with last on the way to connection.  As an example, let's say the user clicked on the body of the notification, then clicked on the phone in settings, but after a certain amount of time (or another signal like closing settings) proceeded no further; we'd count that as INTERACTION_SETTINGS_PHONE_LIST or something.  My guess is we will end up seeing is an exponential decay graph over the buckets, but there might be a particular bucket that has more drop-off than others.

Actually, I think my suggestion is not necessary.  Assuming all users complete all steps successfully (for each possible path), the histogram buckets should be comparable in size, and if there are any particular drop-off points, we should be able to see that as a significant decline from one bucket to the next.

Jordy will be adding metrics for clicking on each type of notification as well as clicking the "connect" button on the "available hosts nearby" notification.

For now, we won't implement the "last action clicked" metric mentioned earlier.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/5a5241642a8e0259ed9c7a847d63a436ce175ca7

commit 5a5241642a8e0259ed9c7a847d63a436ce175ca7
Author: Jordy Greenblatt <jordynass@chromium.org>
Date: Mon Feb 12 23:01:53 2018

[CrOS Tether] Add metrics tracking user interactions with notifications.

The CL counts 5 user/notification interactions, namely when the user
clicks the
  (0) body of the notification shown when a single Tether host is
      present,
  (1) body of the notification shown when multiple Tether hosts are
      present,
  (2) body of the notification shown when setup is required,
  (3) body of the notification shown when a connection attempt failed,
      and
  (4) button on the notification that a single Tether host is present
with the numbers corresponding to the enum values used for recording the
interaction type.

Bug:  808499 
Change-Id: I16594049cb9d5442069963691b3d41b7e56f2b95
Reviewed-on: https://chromium-review.googlesource.com/909804
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#536228}
[modify] https://crrev.com/5a5241642a8e0259ed9c7a847d63a436ce175ca7/chrome/browser/ui/ash/network/tether_notification_presenter.cc
[modify] https://crrev.com/5a5241642a8e0259ed9c7a847d63a436ce175ca7/chrome/browser/ui/ash/network/tether_notification_presenter.h
[modify] https://crrev.com/5a5241642a8e0259ed9c7a847d63a436ce175ca7/chrome/browser/ui/ash/network/tether_notification_presenter_unittest.cc
[modify] https://crrev.com/5a5241642a8e0259ed9c7a847d63a436ce175ca7/tools/metrics/histograms/enums.xml
[modify] https://crrev.com/5a5241642a8e0259ed9c7a847d63a436ce175ca7/tools/metrics/histograms/histograms.xml