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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_metric_values(self, group_name):
 """ Get the faked metric values for a metric group, by its metric group name. The result includes all metric object values added earlier for that metric group name, using :meth:`~zhmcclient.FakedMetricsContextManager.add_metric_object_values` i.e. the metric values for all resources and all points in time that were added. Parameters: group_name (:term:`string`):
 Name of the metric group. Returns: iterable of :class:`~zhmclient.FakedMetricObjectValues`: The metric values for that metric group, in the order they had been added. Raises: ValueError: Metric values for this group name do not exist. """

        if group_name not in self._metric_values:
            raise ValueError("Metric values for this group name do not "
                             "exist: {}".format(group_name))
        return self._metric_values[group_name]