Isn't this already done as part of the TechnologyState metric?

The FeatureState metric doesn't granularly capture how many times users have enabled or disabled the feature. It might be possible to infer this proposed metric from the FeatureState metric, but having this as its own explicit metric allows us to at a glance determine if users are enabling or disabling Tether.

Taking this off Ryan's plate.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/a5837cf4f67606dc5ae3cbfb2139a85af35c7fe1

commit a5837cf4f67606dc5ae3cbfb2139a85af35c7fe1
Author: Kyle Horimoto <khorimoto@google.com>
Date: Fri Oct 13 14:48:31 2017

[CrOS Tether] Add metrics for enabling/disabling Instant Tethering.

There are two metrics:
(1) Toggle metric: will be logged each time the setting is toggled from enabled
to disabled or vice versa. This provides us statistics on how often the
preference is changed.
(2) Startup metric: will be logged each time the user logs in. This provides us
statistics on what percentage of users have the preference enabled.

Bug:  772474 , 672263
Change-Id: Ie0a405d6d1031d3e70e4d8c8b71c3533e04748fd
Reviewed-on: https://chromium-review.googlesource.com/713035
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Ryan Hansberry <hansberry@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#508700}
[modify] https://crrev.com/a5837cf4f67606dc5ae3cbfb2139a85af35c7fe1/chrome/browser/chromeos/tether/tether_service.cc
[modify] https://crrev.com/a5837cf4f67606dc5ae3cbfb2139a85af35c7fe1/chrome/browser/chromeos/tether/tether_service_unittest.cc
[modify] https://crrev.com/a5837cf4f67606dc5ae3cbfb2139a85af35c7fe1/tools/metrics/histograms/histograms.xml

Looks like this didn't make the M-63 cut. Requesting merge into M-63.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/0b5055129eb350d725d81793dfa7463ec6a24f09

commit 0b5055129eb350d725d81793dfa7463ec6a24f09
Author: Kyle Horimoto <khorimoto@google.com>
Date: Mon Oct 16 22:20:26 2017

[CrOS Tether] Add metrics for enabling/disabling Instant Tethering.

There are two metrics:
(1) Toggle metric: will be logged each time the setting is toggled from enabled
to disabled or vice versa. This provides us statistics on how often the
preference is changed.
(2) Startup metric: will be logged each time the user logs in. This provides us
statistics on what percentage of users have the preference enabled.

TBR=khorimoto@google.com

(cherry picked from commit a5837cf4f67606dc5ae3cbfb2139a85af35c7fe1)

Bug:  772474 , 672263
Change-Id: Ie0a405d6d1031d3e70e4d8c8b71c3533e04748fd
Reviewed-on: https://chromium-review.googlesource.com/713035
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Ryan Hansberry <hansberry@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#508700}
Reviewed-on: https://chromium-review.googlesource.com/722116
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/branch-heads/3239@{#21}
Cr-Branched-From: adb61db19020ed8ecee5e91b1a0ea4c924ae2988-refs/heads/master@{#508578}
[modify] https://crrev.com/0b5055129eb350d725d81793dfa7463ec6a24f09/chrome/browser/chromeos/tether/tether_service.cc
[modify] https://crrev.com/0b5055129eb350d725d81793dfa7463ec6a24f09/chrome/browser/chromeos/tether/tether_service_unittest.cc
[modify] https://crrev.com/0b5055129eb350d725d81793dfa7463ec6a24f09/tools/metrics/histograms/histograms.xml