[Messenger] Add doc for default routing for messages

[clemherreman]

Hello,

I recently got bit on production after deploying a code change. We created a new FooMessage & FooHandler, but didn't update the messenger.yaml to include routing information.

This caused FooMessage to be directly handled by the HTTP process that dispatched the message, instead of the CLI, consumer process of Messenger. This lead to various issues: missing env var in HTTP context, lack of monitoring/stats as we only looks for messages metrics in the consumer container, not the api one :)

Looking at the code, I discovered you may define a default transport, but couldn't find it in the doc.

[Nyholm]

👍🏽
I have not verified if this is a feature or not. But assuming you are correct; this feature definitely needs documentation.

[artyuum]

I've been doing this in few of our projects and it works well. I think I heard about this on the Symfony slack and not in the documentation, so it's a nice addition.

[HeahDude]

Many thanks for that PR!

[HeahDude]

Suggested change

	matched under ``routing`` will still be handled immediately, i.e. synchronously.
	matched under ``routing`` will still be handled immediately (e.g. synchronously).

[clemherreman]

Before merging that PR, I'd like to add something.

Using * as a default transport means all "system" messages, sent by the framework itself, will use that transport. That include Symfony\Component\Mailer\Messenger\SendEmailMessage, as the Mailer component detects that the Messenger component is there, and uses it to send email instead of sending it directly.

This caused issues on my project, because SendEmailMessage was not always serializable, if you add an attachment to your email using a PHP resource/stream.

I am not sure how to convey that complex information on the Symfony doc though. Maybe someone with more experience on writing could improve that?

[javiereguiluz]

Thanks Clement! I reworded your last comment as follows while merging: javiereguiluz@5781a94