refactor: make stop_process_timer return duration

[morph-dev]

What was wrong?

The StorageMetricsReporter doesn't return elapsed duration when stop_process_timer is called, which can be useful in some situations.

This is a followup from #1295.

How was it fixed?

Make stop_process_timer return elapsed duration and use it to simplify the logic for updating pruning strategy.

To-Do

• Clean up commit history and use conventional commits.

[njgheorghita]

🚢