code: func (a Sources) MarshalBinary() ([]byte, error) {
  	var pb internal.Measurements
  	pb.Items = make([]*internal.Measurement, len(a))
  	for i, source := range a {
  		pb.Items[i] = encodeMeasurement(source.(*Measurement))
  	}
  	return proto.Marshal(&pb)
  }
docs: // MarshalBinary encodes a list of sources to a binary format.