code: def metadata(proto_dataset_uri, relpath_in_dataset, key, value):
      
      proto_dataset = dtoolcore.ProtoDataSet.from_uri(
          uri=proto_dataset_uri,
          config_path=CONFIG_PATH)
      proto_dataset.add_item_metadata(
          handle=relpath_in_dataset,
          key=key,
          value=value)
docs: Add metadata to a file in the proto dataset.