pick() will only include the specified fields from the message when sending down the pipeline. It currently only works for top level fields (i.e. address.street would not work).
pick({"fields": ["name"]})message in
{ "_id": 0, "name": "transporter", "type": "function" }config
pick({"fields":["_id", "name"]})message out
{ "_id": 0, "name": "transporter" }