Keep runtime configuration lookup cheap and bounded

[lan17]

Priority

P2 — Medium — important hardening or API work after the safety foundations.

Problem

Every enabled invocation awaits cacheConfigProvider before local lookup and before exact-key single-flight. A slow provider can erase local-hit benefits, multiply config-store load across concurrent callers, or hang requests before coalescing can help. Even the no-custom-provider path crosses an async abstraction.

Evidence:

• Provider before cache and single-flight
• README delegates memoization to callers
• Current single-flight boundary

Open decisions

Choose whether memoization/coalescing is built in or opt-in, including cache granularity, refresh/invalidation, acceptable staleness, and timeout semantics.

Acceptance criteria

• Add a no-custom-provider fast path using the definition's static default policy.
• Define timeout and fail-open behavior.
• Add provider call, latency, timeout, and outcome metrics.
• Test concurrent same-key calls and live config changes.
• Benchmark no provider, synchronous provider, and asynchronous provider.
• Do not silently change dynamic-provider freshness semantics.