Consider moving metadata into separate channel in metrological agents

[BjoernLudwigPTB]

At the moment we are appending one set of metadata for each packet send during a call of MetrologicalMonitorAgent's on_received_message and in the agents' agent_loop. We might as well transfer metadata over a separate channel metadata corresponding to the procedure in tutorial 3. This would allow for dropping the overloaded pack_data. We would need to change the call of send_output and on_received_message and maybe other places.