feat(storage)!: remove CacheMeterProvider; use otelcache.RegisterCache

[achille-roussel]

Stacked on #93. Once that merges, rebase this onto main.

Summary

• Remove storage.CacheMeterProvider option, the metricsRegistration field on Cache, and the OTel metric import from storage/cache.go. The storage package no longer depends on go.opentelemetry.io/otel/metric.
• Delete storage/cache_metrics.go (148 lines) — replaced by a single helper in otelcache.
• Add otelcache.RegisterCache(provider, *storage.Cache, attrs...) that wires all three sub-caches (objects/infos/pages) under one callback. The callback snapshots cache.Stat() once per collection so the three kinds always report a coherent view.
• Refactor otelcache.Register to share instrument creation with RegisterCache via internal newInstruments / instruments.observe helpers.

Why

PR #93 introduced otelcache to decouple storage/file.Cache from OTel. This finishes the same decoupling for the in-memory storage.Cache.

Breaking change

storage.CacheMeterProvider is removed.

// before
cache := storage.NewCache(storage.CacheMeterProvider(provider))

// after
cache := storage.NewCache()
reg, _ := otelcache.RegisterCache(provider, cache)
defer reg.Unregister()

Metric names, units, descriptions, and the storage.cache.kind attribute are unchanged, so existing dashboards keep working. The storage.cache.id attribute that the old option auto-derived from the cache pointer must now be supplied by the caller (or omitted).

Test plan

• go test ./storage/...
• go vet ./...
• Confirm storage no longer imports otel/metric: go list -f '{{ join .Imports "\n" }}' ./storage | grep otel/metric returns empty.
• Migrated metric assertions from the deleted cache_metrics_test.go to storage/otelcache/cache_test.go — same 18 (kind, metric) combinations verified.

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