code: def _ParseFieldsMetadata(self, structure):
      
      fields = structure.fields.split(' ')
  
      log_line_structure = pyparsing.Empty()
      if fields[0] == 'date' and fields[1] == 'time':
        log_line_structure += self.DATE_TIME.setResultsName('date_time')
        fields = fields[2:]
  
      for member in fields:
        log_line_structure += self._LOG_LINE_STRUCTURES.get(member, self.URI)
  
      updated_structures = []
      for line_structure in self._line_structures:
        if line_structure[0] != 'logline':
          updated_structures.append(line_structure)
      updated_structures.append(('logline', log_line_structure))
      # TODO: self._line_structures is a work-around and this needs
      # a structural fix.
      self._line_structures = updated_structures
docs: Parses the fields metadata and updates the log line definition to match.
  
      Args:
        structure (pyparsing.ParseResults): structure parsed from the log file.