I'd say we should log when any of these occur:

*Disabled because BLE wasn't fully supported (i.e., the Link case)
*Disabled because the user did not have any synced tether host devices
*Disabled because Bluetooth was turned off
*Disabled because of policy
*Disabled because of preference (cellular or tether)

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/2b0fd1c60f6216d93ad132d68cc873bc172b0ed3

commit 2b0fd1c60f6216d93ad132d68cc873bc172b0ed3
Author: Ryan Hansberry <hansberry@chromium.org>
Date: Thu Aug 10 17:05:37 2017

Add metrics for Tether being enabled or disabled, and why.

Bug:  748306 , 672263
Change-Id: I2c2ca9f70205fc6389c759938eedc487b9115772
Reviewed-on: https://chromium-review.googlesource.com/595149
Commit-Queue: Ryan Hansberry <hansberry@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#493433}
[modify] https://crrev.com/2b0fd1c60f6216d93ad132d68cc873bc172b0ed3/chrome/browser/chromeos/tether/tether_service.cc
[modify] https://crrev.com/2b0fd1c60f6216d93ad132d68cc873bc172b0ed3/chrome/browser/chromeos/tether/tether_service.h
[modify] https://crrev.com/2b0fd1c60f6216d93ad132d68cc873bc172b0ed3/chrome/browser/chromeos/tether/tether_service_unittest.cc
[modify] https://crrev.com/2b0fd1c60f6216d93ad132d68cc873bc172b0ed3/tools/metrics/histograms/enums.xml
[modify] https://crrev.com/2b0fd1c60f6216d93ad132d68cc873bc172b0ed3/tools/metrics/histograms/histograms.xml

Approving merge to M61 Chrome OS.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/4b7dadb389a2460867d2bbd3ab4370b9b532c4d1

commit 4b7dadb389a2460867d2bbd3ab4370b9b532c4d1
Author: Kyle Horimoto <khorimoto@google.com>
Date: Thu Aug 10 18:15:57 2017

Add metrics for Tether being enabled or disabled, and why.

TBR=hansberry@chromium.org

(cherry picked from commit 2b0fd1c60f6216d93ad132d68cc873bc172b0ed3)

Bug:  748306 , 672263
Change-Id: I2c2ca9f70205fc6389c759938eedc487b9115772
Reviewed-on: https://chromium-review.googlesource.com/595149
Commit-Queue: Ryan Hansberry <hansberry@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#493433}
Reviewed-on: https://chromium-review.googlesource.com/610891
Cr-Commit-Position: refs/branch-heads/3163@{#441}
Cr-Branched-From: ff259bab28b35d242e10186cd63af7ed404fae0d-refs/heads/master@{#488528}
[modify] https://crrev.com/4b7dadb389a2460867d2bbd3ab4370b9b532c4d1/chrome/browser/chromeos/tether/tether_service.cc
[modify] https://crrev.com/4b7dadb389a2460867d2bbd3ab4370b9b532c4d1/chrome/browser/chromeos/tether/tether_service.h
[modify] https://crrev.com/4b7dadb389a2460867d2bbd3ab4370b9b532c4d1/chrome/browser/chromeos/tether/tether_service_unittest.cc
[modify] https://crrev.com/4b7dadb389a2460867d2bbd3ab4370b9b532c4d1/tools/metrics/histograms/enums.xml
[modify] https://crrev.com/4b7dadb389a2460867d2bbd3ab4370b9b532c4d1/tools/metrics/histograms/histograms.xml