def snapshot(self):
        """
        Return a nested dictionary snapshot of all metrics and their
        values at this time. Example:
        {
            'category': {
                'metric1_name': 42.0,
                'metric2_name': 'foo'
            }
        }
        """
        return dict((category, dict((name, metric.value())
                                    for name, metric in list(metrics.items())))
                    for category, metrics in
                    list(self._store.items()))