code: public Object fieldToPig(FieldDescriptor fieldDescriptor, Object fieldValue) {
      if (fieldValue == null) {
        // protobufs unofficially ensures values are not null. just in case:
        return null;
      }
      if (fieldDescriptor.getType() == FieldDescriptor.Type.MESSAGE) {
        return messageToTuple(fieldDescriptor, fieldValue);
      } else {
        return singleFieldToTuple(fieldDescriptor, fieldValue);
      }
    }
docs: Returns either {@link #messageToTuple(FieldDescriptor, Object)}
  or {@link #singleFieldToTuple(FieldDescriptor, Object)} depending
  on whether the field is a Message or a simple field.