Add logging.access.sample_rate config (0.0-1.0) for deterministic
request sampling. 5xx errors are always logged; 2xx/3xx/4xx follow
the configured rate. Uses atomic.Uint64 counter for lock-free,
zero-allocation sampling decisions.
Includes test updates to verify:
- sample_rate=1.0 logs all requests
- sample_rate=0.0 logs only 5xx
- 5xx are always logged regardless of rate
xfy
047e033af5