'Return a nested dictionary snapshot of all metrics and their
values at this time. Example:
\'category\': {
\'metric1_name\': 42.0,
\'metric2_name\': \'foo\''

def snapshot(self):

    return dict(((category, dict(((name, metric.value()) for (name, metric) in list(metrics.items())))) for (category, metrics) in list(self._store.items())))
