I am fully aware of the implementation and future plans, I made both of them 
Yes, but the plan is to make it Erlang logger formatter, as you have noticed.
Internally, yes, from the user viewpoint, no. This is required for the compatibility. So in Elixir 1.10 it will be changed before returning in Logger.metadata/0.
Yes, as metadata is for something completely different. Using your example in metadata_logger_plug the status, body length, and response status should be part of the report, not metadata. Metadata is more about, well, meta data like: version, log position, module, function, application name, etc.
Well, partially. It will work for that particular formatter, but in general it will fail (also Dialyzer will complain) as you break contract about type of the accepted data. So that single formatter will work (because it relies on undefined behaviour), but any other will fail, and this will fail with parsing Erlang reports, as these are handled before these end in your translator.
