code: def add_metric_group_definition(self, definition):
          
          assert isinstance(definition, FakedMetricGroupDefinition)
          group_name = definition.name
          if group_name in self._metric_group_defs:
              raise ValueError("A metric group definition with this name "
                               "already exists: {}".format(group_name))
          self._metric_group_defs[group_name] = definition
          self._metric_group_def_names.append(group_name)
docs: Add a faked metric group definition.
  
          The definition will be used:
  
          * For later addition of faked metrics responses.
          * For returning the metric-group-info objects in the response of the
            Create Metrics Context operations.
  
          For defined metric groups, see chapter "Metric groups" in the
          :term:`HMC API` book.
  
          Parameters:
  
            definition (:class:~zhmcclient.FakedMetricGroupDefinition`):
              Definition of the metric group.
  
          Raises:
  
            ValueError: A metric group definition with this name already exists.