feat(metrics): enhance cache efficiency metrics

[JaredTan95]

Part of #617.

Completes Sub-issue 3: Cache Efficiency Metrics Enhancement.

The index previously exposed aggregate admission/eviction counters without backend attribution or explicit cache efficiency gauges. This change adds a backend label to the existing counters and introduces kvcache_index_entries and kvcache_index_hit_rate gauges through the existing metrics registry.

• Distinguish in_memory, cost_aware_memory, redis, and valkey backends, with unknown for custom index implementations. Metrics logging continues to aggregate admissions and evictions across backend series.
• Report entry counts for the in-memory and cost-aware-memory indexes using inexpensive size queries. Redis/Valkey gauges are not emitted because these backends do not expose an inexpensive, accurate request-key count.
• Update the gauges after index operations and record lookup hit metrics synchronously. The hit-rate gauge uses the existing process-wide cumulative lookup_hits_total / lookup_requests_total values; it is not an independently calculated per-backend hit ratio.
• Test collector registration, gauge updates, and isolation of admission/eviction counter series between backends.

Existing counter metric names are preserved; consumers now receive backend-labeled series and can use sum(...) to retain aggregate views.

Validation:

• go test ./pkg/kvcache/metrics ./pkg/kvcache/kvblock
• go test -race ./pkg/kvcache/kvblock
• git diff --check

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