The akka.zeromq.Subscribe option establishes a new message filter on a akka.zeromq.SocketType.Pub socket.
Newly created akka.zeromq.SocketType.Sub sockets filter out all incoming messages,
therefore you should send this option to establish an initial message filter.
An empty payload of length zero will subscribe to all incoming messages.
A non-empty payload will subscribe to all messages beginning with the specified prefix.
Multiple filters may be attached to a single akka.zeromq.SocketType.Sub socket,
in which case a message will be accepted if it matches at least one filter.
the topic to subscribe to