Enriched item or channel level metadata

[splatch]

I come across couple of bindings and I've found that there is no clear way of expressing last update, last error time or last error itself on a given channel or for given thing. The only way how developer can react is marking thing offline however in some cases a partial configuration (when single channel is wrong) is still fine. Marking a channel as a faulty could also increase user experience.

As discussed in #1298 there are a cases when commands can be sent with additional flags, but in several places they can be also received with a flag (BACnet and command priority for example). If attributes will fly over then its fin, but BACnet has also fault flag which is related to single object such analog input or output and AFAIK can change at any time.

I know that we can track items and states registered there, however quite often binding developers decide to provide meta channels which serve very similar purpose.

This issue is registered to discuss the idea and see if there is a ground for such functionality or not. Part of the reason why I bring it here is to ie. determine a state of radio device. Knowing that I have something which is updated for example every minute I can determine if my thing went offline. I had to implement such functionality in wmbus binding but I believe it is fairly common case for other places too.