async/rate_limiter_utils library

Token-bucket rate limiter — roadmap #670.

Smooths a burst of work to a sustainable average rate: tokens refill continuously at tokensPerSecond up to a capacity (the burst ceiling), and each unit of work spends tokens. This is the non-blocking core primitive — tryAcquire returns allow/deny immediately and timeUntilAvailable says how long until a denied request would succeed, so the caller decides whether to drop, queue, or delay. No Timer and no wall-clock coupling: time is read through an injectable now closure, making refill behavior fully deterministic under test.

Classes

TokenBucketRateLimiter

A token bucket: capacity tokens max, refilling at tokensPerSecond.