code: public static Message buildFileHeader(long length) {
      ZabMessage.FileHeader header = ZabMessage.FileHeader.newBuilder()
                                               .setLength(length).build();
      return Message.newBuilder().setType(MessageType.FILE_HEADER)
                                 .setFileHeader(header).build();
    }
docs: Creates a FILE_HEADER message.
  
  @param length the lenght of the file.
  @return a protobuf message.