code: def _add_message_field(self, field_name, value, params):
      
      if '.' not in field_name:
        params[field_name] = value
        return
  
      root, remaining = field_name.split('.', 1)
      sub_params = params.setdefault(root, {})
      self._add_message_field(remaining, value, sub_params)
docs: Converts a . delimitied field name to a message field in parameters.
  
      This adds the field to the params dict, broken out so that message
      parameters appear as sub-dicts within the outer param.
  
      For example:
        {'a.b.c': ['foo']}
      becomes:
        {'a': {'b': {'c': ['foo']}}}
  
      Args:
        field_name: A string containing the '.' delimitied name to be converted
          into a dictionary.
        value: The value to be set.
        params: The dictionary holding all the parameters, where the value is
          eventually set.