When working with multiple Messenger buses, it's common to restrict a handler to certain bus(es) using the tag properties. This is useful if you have handlers that all belong to specific buses, but becomes more complicated when there are more combinations of (default and non-default) buses and handlers.

For example if you have a bus to handle new messages and want to introduce a bus that only handles a migrations process' messages in the background. If the handlers for "new messages" should keep handling only those, then you'd suddenly need to make that explicit for all those handlers, because otherwise they'd be added to the "migration process" bus too. And if a third party bundle also registers message handlers, you'd need to restrict those to the specific bus(es) too, if even possible.

This can be improved (IMHO) with a simple addition, by adding a configuration value for the messenger configuration (framework.messenger.buses[].default_handlers) that determines if the bus is added by default to handlers. This then stores this config in a tag property, which can be read in the MessengerPass. The change would be fully backwards compatible. I'd first like to know if this is in line with the Messenger component's way of working and something that could be added. I can add/fix tests, codestyle etc after discussion.