My gut feeling is that kUnknown mostly means "no network connected" or "I forgot to initialize the connection type variable."  I wasn't able to trigger this by hand, but I didn't try very hard either.

I would prefer to have a separate kDisconnected constant to indicate that update_engine checked with shill, and shill is reporting no system connectivity.  kUnknown feels more like "internal error" to me.

It looks like UpdateAttempter::ProcessingDone() can call ScheduleErrorEventAction() which calls PayloadState::UpdateFailed() which calls CollectAndReportAttemptMetrics().  ProcessingDone() will skip this step if status_ == UpdateStatus::REPORTING_ERROR_EVENT or if code == ErrorCode::kSuccess.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/9a81ea9cb577f7e1ed31e23482532242c313ec43

commit 9a81ea9cb577f7e1ed31e23482532242c313ec43
Author: Colin Howes <chowes@google.com>
Date: Fri Nov 16 19:13:21 2018

Add disconnected connection type for Update Engine

Add a network disconnected connection type to the Update Engine
connection type histogram.

BUG= chromium:660283 

Change-Id: Icc93ec2f49073566dcb49d6abcd70bc535aca176
Reviewed-on: https://chromium-review.googlesource.com/c/1235282
Reviewed-by: Xiaochu Liu <xiaochu@chromium.org>
Commit-Queue: Colin Howes <chowes@google.com>
Cr-Commit-Position: refs/heads/master@{#608884}
[modify] https://crrev.com/9a81ea9cb577f7e1ed31e23482532242c313ec43/tools/metrics/histograms/enums.xml

The following revision refers to this bug:
  https://chromium.googlesource.com/aosp/platform/system/update_engine/+/c9e98d6995bdd22de37f18e6b175775939d6159d

commit c9e98d6995bdd22de37f18e6b175775939d6159d
Author: Colin Howes <chowes@google.com>
Date: Sun Nov 18 15:11:07 2018

update_engine: Add disconnected network type for metrics reporting

On a failed update, correctly report that the network is disconnected
rather than reporting an "unknown" network type, which is set when the
connection type cannot be determined based on the properties of a given
service path.

BUG= chromium:660283 
TEST=Added relevant unittests. Also tested manually by updating via cros
flash, unplugging Ethernet after the transfer, and killing the
dev_server. Metrics sends "disconnected" as expected.

Change-Id: Ia681b769208b67fb5c14d41a646d816a42ccf33e
Reviewed-on: https://chromium-review.googlesource.com/1231700
Commit-Ready: Colin Howes <chowes@google.com>
Tested-by: Colin Howes <chowes@google.com>
Reviewed-by: Amin Hassani <ahassani@chromium.org>

[modify] https://crrev.com/c9e98d6995bdd22de37f18e6b175775939d6159d/metrics_utils.cc
[modify] https://crrev.com/c9e98d6995bdd22de37f18e6b175775939d6159d/metrics_constants.h
[modify] https://crrev.com/c9e98d6995bdd22de37f18e6b175775939d6159d/connection_manager.cc
[modify] https://crrev.com/c9e98d6995bdd22de37f18e6b175775939d6159d/connection_manager.h
[modify] https://crrev.com/c9e98d6995bdd22de37f18e6b175775939d6159d/metrics_utils_unittest.cc
[modify] https://crrev.com/c9e98d6995bdd22de37f18e6b175775939d6159d/connection_manager_unittest.cc
[modify] https://crrev.com/c9e98d6995bdd22de37f18e6b175775939d6159d/update_manager/boxed_value_unittest.cc
[modify] https://crrev.com/c9e98d6995bdd22de37f18e6b175775939d6159d/connection_utils.cc
[modify] https://crrev.com/c9e98d6995bdd22de37f18e6b175775939d6159d/connection_utils.h