Add last_seen attributes to all Device Trackers

Use case: you see the GPS location of your girlfriend, but you wonder if it is a stale location (lost data, shutdown due to battery) or a fresh one. This could be easily added by adding an attribute ‘last_seen’ or similar, which just adds the timestamp when homeassistant received the location.

As an example, the google_maps platform provides the last_seen attribute, this attribute is provided by google maps and it’s simply exposed.

After looking in the code it seems rather simple to add this (have no dev env setup though, so not sure I’ll be able to find time for it).
Some remaining questions:

  • do we make this a standard/required attribute for all device trackers, or add it just to a few
  • do we call it ‘last_updated’/… or keep the ‘last_seen’ from google maps.