We actually do already track advertisement unregistration, in a very similar manner to registration.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d585a0ee966bc03d8a66eecbeaf12b589944bdf5

commit d585a0ee966bc03d8a66eecbeaf12b589944bdf5
Author: Kyle Horimoto <khorimoto@google.com>
Date: Tue Oct 24 17:39:36 2017

[CrOS Tether] Add metrics for stopping discovery sessions.

Bug:  777027 , 672263
Change-Id: I2dc1447601b6c3c65e2d8390e917b20c8fff4bf5
Reviewed-on: https://chromium-review.googlesource.com/734208
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Ryan Hansberry <hansberry@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#511199}
[modify] https://crrev.com/d585a0ee966bc03d8a66eecbeaf12b589944bdf5/chromeos/components/tether/ble_synchronizer.cc
[modify] https://crrev.com/d585a0ee966bc03d8a66eecbeaf12b589944bdf5/chromeos/components/tether/ble_synchronizer.h
[modify] https://crrev.com/d585a0ee966bc03d8a66eecbeaf12b589944bdf5/chromeos/components/tether/ble_synchronizer_unittest.cc
[modify] https://crrev.com/d585a0ee966bc03d8a66eecbeaf12b589944bdf5/tools/metrics/histograms/histograms.xml

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/e634d967c202e2b3e9a62dd732013b222b1c4289

commit e634d967c202e2b3e9a62dd732013b222b1c4289
Author: Kyle Horimoto <khorimoto@google.com>
Date: Wed Oct 25 17:55:18 2017

[CrOS Tether] Add metrics for stopping discovery sessions.

TBR=khorimoto@google.com

(cherry picked from commit d585a0ee966bc03d8a66eecbeaf12b589944bdf5)

Bug:  777027 , 672263
Change-Id: I2dc1447601b6c3c65e2d8390e917b20c8fff4bf5
Reviewed-on: https://chromium-review.googlesource.com/734208
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Ryan Hansberry <hansberry@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#511199}
Reviewed-on: https://chromium-review.googlesource.com/737419
Cr-Commit-Position: refs/branch-heads/3239@{#226}
Cr-Branched-From: adb61db19020ed8ecee5e91b1a0ea4c924ae2988-refs/heads/master@{#508578}
[modify] https://crrev.com/e634d967c202e2b3e9a62dd732013b222b1c4289/chromeos/components/tether/ble_synchronizer.cc
[modify] https://crrev.com/e634d967c202e2b3e9a62dd732013b222b1c4289/chromeos/components/tether/ble_synchronizer.h
[modify] https://crrev.com/e634d967c202e2b3e9a62dd732013b222b1c4289/chromeos/components/tether/ble_synchronizer_unittest.cc
[modify] https://crrev.com/e634d967c202e2b3e9a62dd732013b222b1c4289/tools/metrics/histograms/histograms.xml