Throttle cache stats writes to avoid per-request option updates

[mehul0810]

Finding

StatsStore::flush() writes the full perform_cache_stats option whenever any stat changes, and page-cache hit/miss/bypass paths call stat increments during normal frontend traffic.

Evidence

• src/Modules/Cache/PageCache.php increments and flushes stats on cache hits/stale hits, misses, lock waits, bypasses, and slow uncached requests.
• src/Modules/Cache/StatsStore.php persists those changes with update_option( 'perform_cache_stats', ... , false ) in flush().

Impact

A busy site can turn otherwise cheap cached page views into frequent database writes. That undermines the page-cache module's main performance goal and can become a release-blocking scalability regression on high-traffic installs.

Suggested fix

Batch or sample observability writes instead of updating the option on every request. Reasonable options include a transient-backed throttle, periodic cron flush, in-memory aggregation per request class, or only writing top-miss/slow maps when thresholds change.

Acceptance criteria

• Cache HIT/STALE/BYPASS traffic does not write perform_cache_stats on every request.
• Observability counters remain useful after batching.
• Existing option keys are preserved for backward compatibility.

[mehul0810]

Adding the latest scalability review note here instead of opening a duplicate issue:

perform_cache_stats also needs to account for concurrent updates. Even after per-request write throttling/sampling, two overlapping requests can read the same stats option, mutate separate in-memory copies, and overwrite each other on update_option().

When implementing this issue, please include the durable telemetry angle in the design:

• Keep the current option key readable for backward compatibility.
• Avoid hot-path database writes on cache HIT/STALE/BYPASS traffic.
• Avoid lost updates under concurrent frontend traffic where practical.
• Consider object-cache increments, Action Scheduler/cron aggregation, or a small indexed storage layer if stats become product-critical.
• Keep observability approximate if that is the pragmatic 1.6.0 choice, but document that it is approximate.

[mehul0810]

Completed in PR #98 against release/1.6.0. Cache stats writes are now sampled and merged to avoid hot-path option updates.