The message provenance.
The event payload.
ReadonlysignalAborts when the sender stops waiting for a reply.
The topic that carries the message.
Refuses the message: the sender's awaited emit() rejects with a
MessageBusError whose code is 'REFUSED'. Ignored, with a warning,
once the reply has already settled.
Optionalmessage: string
A message delivered to an event topic responder.