{"type": "progress", "data": {"chunk": 3, "total": 100}}. The wire type is
the snake_case form of the class name. The class docstring documents the message in the published
schema. A message with no fields is valid and is
the right shape for a pure signal.
Send one to every connected client with await self.send(message), or to a single client with
await client.send(message). Returning one from an @event handler makes it that command’s
correlated reply. See Events & Messages.