This PR introduces a new `DurationCounter64` class that extends `Counter64` to better handle duration-based metrics. The class provides a type-safe way to track elapsed time with specific duration units.

Key changes:

• Added `DurationCounter64<T>` template class that extends `Counter64` with duration-specific functionality
• Updated `ScopedTimerMetric` to work with the new duration counter type
• Modified the `ServerStatusMetricPolicySelection` template to support all `Counter64` derived types
• Refactored the extension stage execution time tracking to use the new duration counter

These changes improve type safety when tracking time-based metrics and make the code more maintainable by ensuring duration units are consistently handled.