Mod. Handle additional aggregator args via **kwargs.
Mod. Add **kwargs to aggregator call + PEP-8 refactor.
Add. UserMetric.METRIC_AGG_METHOD_KWARGS attribute to rate aggregators
Mod. clean up for PEP-8 compliance.
Add. Defaults for aggregator `header` and `kwarg` attributed in `aggregator` method.
Fix. included **kwargs in aggregator wrapper for verifying metric types.
Add. rate aggregator for edit_rate metric.