Function describe_message 

pub fn describe_message(msg: &Message) -> String

Render a message as its variant and identifying fields, never its payload.

Prod timeout errors named neither the message nor the agent, so 270 ERROR lines over two weeks could not be chased back to a cause (CLOUDPUB-1511). Every arm is spelled out on purpose - the Protocol enums take no _ default arm, so a new variant has to be described here rather than silently logging as “unknown”.

Credentials are never rendered: AgentHello carries a token and a password and AgentAck is a bare token, so those arms name the variant and the agent, nothing else.