Implement sliding window observer for kernel metrics

## Goal

Implement timestamp-based cleanup mechanism for kernel metrics.

## Requirements

- Track last collection timestamp for each kernel metric
- Configurable cleanup window (multiple of 5 minutes, the metric collection interval)
- Auto-remove metrics not updated within the window
- Integrate with existing StatContext

## Implementation

Add sliding window observer to `stats.py`:

- New class or method to track timestamps
- Configuration parameter for window size
- Periodic cleanup task

## Dependencies

Should be implemented after bug fix (BA-3521 subtask 1).

JIRA Issue: BA-3523