Normalize (and imo improve) periodic statistics collection in MAD. (#179)

* Normalize (and imo improve) periodic statistics collection in MAD.

* Updated as per feedback. Still needs the period tag injection.

* Added '_period' (configurable name) as dimension to all PeriodicStatisticsSink emitted metrics. Removed global key and emitting metrics when no data is flowing.

* Used

Author: vjkoskela