Correct encoding and decoding of "message/x.rsocket.routing.v0"

[rstoyanchev]

Composite metadata is now supported as of M3 (see #22798) with the String route added as a "message/x.rsocket.routing.v0" entry in the composite metadata, plus the option to add more metadata entries. The route is still encoded as a simple String. That needs to be changed when rsocket-core provides an API for that.

[linux-china]

RoutingMetadata implemented on https://github.com/rsocket/rsocket-java/tree/develop/rsocket-core/src/main/java/io/rsocket/metadata
it's tagging, should some reserved tags be introduced for routing?

[rstoyanchev]

@linux-china, currently we simply take the first tag value. Is this too simplistic perhaps, and is there a more idiomatic way, e.g. one tag with the some name (e.g. "route" or "mapping") + 2nd tag with the actual route value?

[linux-china]

Now I think the first tag is ok, and developers can put URI string as tag value, and still flexible.