Feature request: abstraction for profile context

Hi,

I'm working on a patch to oslo.messaging that will integrate osprofiler for tracing across RPC calls:

https://review.openstack.org/#/c/623254/

Ilya makes a good observation about accessing the current context:

https://review.openstack.org/#/c/623254/1/oslo_messaging/rpc/client.py@129

Ideally oslo.messaging shouldn't have to explicitly construct the components of osprofiler's context as I have done in the patch:

        # send profiler state to the server for tracing the RPC call
        if profiler:
            prof = profiler.get()
            if prof:
                msg['osprofiler'] = {
                    'hmac_key': prof.hmac_key,
                    'base_id': prof.get_base_id(),
                    'parent_id': prof.get_id()
                }

Instead the context should be opaque as far as oslo.messaging is concerned - osprofiler should provide it in the correct serialized form for including in the messages meta-data.

Would it be possible to provide a new profiler access method that returns a pre-built context as a map rather than have oslo.messaging pick out the values explicitly? Then oslo.messaging wouldn't care what's in the context. like this:

        # send profiler state to the server for tracing the RPC call
        if profiler:
            prof = profiler.get()
            if prof:
                msg['osprofiler'] = prof.get_context()

And on the server side we can easily set the state (as we do in the patch):

       ctxt = msg.get('osprofiler')
       if ctxt is not None:
           profiler.init(**ctxt)