phase_stats aggregation should run in the providers directly and not in separate file

[ArneTR]

Currently the phase_stats are aggregated through the file tools/phase_stats.py.

This mechanism from an ownership point of view should run in the metric_providers and not in a separate file.

Rationale is that only the metric_providers know how to sum up their own stats and thus at least a routine from their instance should be called.

This task is a refactoring to bring this functionality to the metric_provider to have all the logic there. This would also be a better fit when thinking of them as plugins as all the logic should be encapsulated in their files and not be external.