Introduce Metric and MetricRepository interfaces.

Reduces CounterRepository, GaugeRepository to marker interfaces
and removes boilerplate from in-memory implementations.
Re-enable tests for MessageCounterHandler.

Other redis tests have already been re-enabled so no point in
ignoring this one.

Fixed typo in GaugeService.
Fix metric names.
Add deleteAll method to MetricRepository.
Refactor Redis repositories.

Pull methods up into base class to remove duplication.