feat(storage/otelcache): expose cache stats via OpenTelemetry

[achille-roussel]

Summary

• New storage/otelcache package: Register(provider, observable, attrs...) wires Stat() storage.CacheStat into OTel observable instruments — gauges for size/limit/entries, counters for hits/misses/evictions. Cache implementations stay free of OTel imports.
• Observable is a one-method interface (Stat() storage.CacheStat); file.Cache satisfies it directly with no shims.
• Fix file.Cache.Stat() to populate Entries from the underlying LRU (it was always zero before).

Why this shape

Implementing go.opentelemetry.io/otel/sdk/metric.Reader would put the library in the SDK's exporter pipeline — the wrong layer, since the application owns the MeterProvider, Views, and exporter setup. Observable instruments registered against the application's MeterProvider are the idiomatic pull model; placing them in a separate package that consumes Stat() keeps the cache types decoupled from OTel.

Usage

fileCache := file.NewCache(tmpdir, size)
reg, _ := otelcache.Register(provider, fileCache,
    attribute.String("storage.cache.kind", "file"))
defer reg.Unregister()

Test plan

• go test ./storage/otelcache/...
• go test ./storage/file/...
• go test ./storage/...
• go vet ./...
• Confirm storage/file has no OTel imports (go list -f '{{ join .Imports "\n" }}' ./storage/file | grep otel → empty)

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