    def _metric_init(self):  # pragma: no cover
        """
        Initialize the metric object as a child, i.e. when it has labels (if any) set.

        This is factored as a separate function to allow for deferred initialization.
        """
        raise NotImplementedError('_metric_init() must be implemented by %r' % self)
Initialize the metric object as a child, i.e. when it has labels (if any) set.

This is factored as a separate function to allow for deferred initialization.
prometheus_client/metrics.py