Code review comment for ~tilmanbaumann/interface-prometheus:reactive_endpoints

Yes please. This is going to be a teaching example, as it is simple enough to comprehend and not need any weird stuff, but complex enough to demonstrate most of the Endpoint stuff (including implementing both provider and consumer; this is the first non-trivial Endpoint I've seen that has done that). The most magic thing is mixing to_publish for implicit JSON encoding with to_publish_raw for the legacy fields that need to be unencoded.

So I'd like to see two minor changes needed to support cross model relations fixed in this branch, so that when people inevitably cut&paste the code they get the good stuff. Some other comments inline too, primarily with setting the old flags for backwards compatibility.