Get the name of the exchange where this message arrived from. The
method will return null if the message did not arrive through an
exchange (e.g. posted directly to a queue).
Reply to the sender of this message with a new message. message should be either a Uint8List, a String, a Map or Iterable.
If message is Map or Iterable it will be encoded as JSON and the appropriate message properties
(content-type and content-encoding) will be set on the outgoing message. Any other message value will
trigger an ArgumentError.