I got it working. Looks like I copy / pasted a bit too enthusiastic from the documentation. (Guess it should be corrected there as well). There was a case mismatch:
def _item_valuespec_rudder_agent():
and
item_spec=_item_ValueSpec_rudder_agent,
Like I said, I copy / pasted from the official documentation and there’s the case difference as well. So now I can finish coding this plugin and upload it to the CheckMK Exchange when finished.