Only apply the metadata backwards-compability shim if necessary

i.e. if the fields query string parameter is empty or missing, or if the fields query string parameter mentions fields requiring backwards-compatibility. If my query looks like `fields=chambers`, it means I just want `chambers`, not all the backwards-compatibility fields. This fix doesn't break backwards-compatibility, since no one (using old, deprecated code) would be setting `fields=chambers` in order to get things like `lower_chamber_name`, etc.