Counters, gauges, percentiles

1. static long p(List<Long> sortedMicros, double q) — the q-th percentile of a SORTED list: index = ceil(q * n) - 1, clamped to >= 0. p(x, 0.5) is the median. 2. static java.util.Map<String, Long> summarize(java.util.List<Long> durations) — sort internally, return {"p50":..., "p95":..., "p99":..., "count": n}. 3. static boolean avgHides(List<Long> durations) — return true iff the MEAN is less than p95 (the classic 'average looks fine' lie). Empty list → false. 4. static boolean cardinalityOk(String labelKey) — true iff labelKey is a CLASS (one of "endpoint", "status", "region"), false for instance labels ("userId", "requestId", "url").

Difficulty: advanced