Flag new heavy hitters: keys that enter the top-k

[zzylol]

Part 6 of the telemetry-functions stack: use case 3, heavy hitters and novelty.

The detector and exact_detector remember each series' top-k keys over the last 10 windows. A key in the current top-k that none of those windows held is new: it is emitted as asap_topk_new and as an OTLP log event. The scenario has 100,000 Zipf routes (exponent 1.1), and at minute 10 a new route takes 15% of traffic. Sketch: Count-Min 4 × 512 with a 10-slot heap per attribute key (per_attribute: [http.route]). With 100,000 keys in 512 columns, collisions are frequent, so the sketch's counts differ visibly from exact.

Result (56-core x86 server, 1-minute windows)

• The sketch top-10 has the same keys in the same order as exact (recall@10 = 1.0). Its counts are 1,000–2,000 too high per key, from collisions: +0.3% for the top route (324,346 vs 323,344) up to +4.9% for rank 10 (26,835 vs 25,581). Count-Min only overestimates.
• /new/hot-route was flagged by both methods in the window it appeared, at rank 2.

The screenshot is from the earlier 200-route, 4 × 2048 setup.

cargo fmt --check, cargo clippy --all-targets --features otap-engine,stream-benchmark -- -D warnings and cargo test --features otap-engine,stream-benchmark pass at this PR's head.