Add actuator support for RSocket

[bclozel]

Since #16021, Spring Boot supports RSocket as a type of server applications (meaning there's a single RSocket server available and no HTTP server).

We should try and expose actuator endpoints in a meaningful way over RSocket.

[spencergibb]

I'm very interested in this as well. It will be interesting to see how this gets done as the routing in Framework is still basic (simple string). Spring Cloud Gateway needs similar functionality.

[philwebb]

I think we might run out of time for this one. I'm going to move it to 2.x for now (@bclozel feel free to move back if you have something in hand already).

[spencergibb]

@bclozel I'd like to chat with you about it. Gateway needs something like this and I'd like to be not far off when I implement it there.

[onobc]

@bclozel I would love to help out on this one.

[bclozel]

@snicoll and I are looking into this issue. We've discussed with the Framework team about this and it seems that:

• there are a lot of open questions still about conventions and best practices when it comes to routes and RSocket interaction models
• we're missing critical infrastructure in Spring Framework to support this, and this might be considered at the same time as a possible functional programming model as well.
• we need to learn more about the use cases behind this and what other pieces would be missing to provide a complete support (we'll get in touch with @spencergibb for that)

We're pushing it back to 2.x as 2.3 will be based on Spring Framework 5.2 and we won't have time to cover all this in the next release.