This adds metrics for
* `ua.command.ack.unknown_version` indicating ACK responses from the
  desktop UA that do not match known, send notifications.
* `ua.command.ack {code: ###}` recording ACK'd messages broken out by
  returned `code`

Closes: SYNC-4456

The notification version is the message_id. The message_id is an
encrypted string containing the uaid, chid, topic or timestamp, etc.
No need to check the channelId, since the version/message_id should
already have it.

… feat/SYNC-4456_track_code