code: def etag(self, etag):
          
          if not isinstance(etag, list):
              etag = [etag]
          for e in etag:
              option = Option()
              option.number = defines.OptionRegistry.ETAG.number
              if not  isinstance(e, bytes):
                  e = bytes(e, "utf-8")
              option.value = e
              self.add_option(option)
docs: Add an ETag option to the message.
  
          :param etag: the etag