code: def __AddAdditionalProperties(self, message, schema, properties):
          
          additional_properties_info = schema['additionalProperties']
          entries_type_name = self.__AddAdditionalPropertyType(
              message.name, additional_properties_info)
          description = util.CleanDescription(
              additional_properties_info.get('description'))
          if description is None:
              description = 'Additional properties of type %s' % message.name
          attrs = {
              'items': {
                  '$ref': entries_type_name,
              },
              'description': description,
              'type': 'array',
          }
          field_name = 'additionalProperties'
          message.fields.append(self.__FieldDescriptorFromProperties(
              field_name, len(properties) + 1, attrs))
          self.__AddImport('from %s import encoding' % self.__base_files_package)
          message.decorators.append(
              'encoding.MapUnrecognizedFields(%r)' % field_name)
docs: Add an additionalProperties field to message.