add metadata to hello requests

[rixmann]

we need to forward a uuid with hello requests (and log it too)
for this purpose structured support from hello would be really nice.

[GalaxyGorilla]

are we talking about the request ids from hello or a uuid for every request-response 'event'?

[rixmann]

we want to provide the uuids in requests from other components.
i think general support for request metadata would be clean.