feat: add support for broker as a client

[getlarge]

i get the idea and agree with it, that would be quite a refactoring of the span management logic.
right now, spans are only focused around the client, changing this to also support the broker as the client would involve several changes, beyond changing the functions signatures.
That would also involve :

• checking when the client is the broker
• storing some attributes into the broker to reuse during span creation
• ensuring that the broker instance is always accessible to manage spans, even though that one should be easy if we consider that the places where “broker is the client” is when calling the broker instance methods
• adding some tests obviously

I would rather do this work in a separate issue/PR.

Originally posted by @getlarge in #3 (comment)