T2.6 — Prometheus metrics for conformance

Status: Pending Review Agent: Kimi Code CLI Started: 2026-05-11T17:45Z Completed: 2026-05-11T17:55Z Commit: 846c98e PRD: ../PRD-relay-conformance.md

What I changed

crates/wzp-relay/src/metrics.rs:
- Updated conformance_violations: IntCounterVec labels from ["violation_type"] to ["tier", "codec_id", "media_type", "verdict"].
- Added conformance_bytes: HistogramVec — packet size distribution, label media_type.
- Added conformance_iat_ms: HistogramVec — inter-arrival time distribution, label media_type.
- Added record_conformance(header, payload_len, iat_ms, violation) helper:
  - Records bytes + IAT histograms on every packet.
  - Increments violation counter (with full labels) only on violations.
crates/wzp-relay/src/room.rs:
- Both run_participant_plain and run_participant_trunked call metrics.record_conformance() on every incoming packet.
- recv_gap_ms (already computed for gap logging) is reused as the IAT measurement.

Why these choices

Histograms are recorded per-packet so operators can see the full distribution of traffic, not just the abusive tail. The media_type label separates audio, video, data, and control traffic without over-labeling (codec_id on histograms would create too many time-series).

The violation counter uses four labels:

tier — "A", "B", or "C" (which conformance check failed)
codec_id — Debug representation (e.g., "Opus24k")
media_type — Debug representation (e.g., "Audio")
verdict — Debug representation of Violation enum

This gives operators enough dimensions to correlate violations with specific codecs and traffic types.

Deviations from the task spec

None.

Verification output

$ cargo test -p wzp-relay conformance
running 10 tests
...(all 10 pass)...

test result: ok. 10 passed; 0 failed; 0 ignored; 0 measured; 76 filtered out; finished in 0.00s

$ cargo test -p wzp-relay
running 86 tests
...(all 86 pass)...

test result: ok. 86 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.01s

Test summary

Tests added: 0 (metrics are exercised indirectly by conformance tests)
Tests modified: 0
wzp-relay test count: 86 (unchanged)
cargo clippy -p wzp-relay --lib: pass (no new warnings)
cargo fmt --all -- --check: pass

Risks / follow-ups

Histogram cardinality is bounded: media_type has 4 values, so conformance_bytes and conformance_iat_ms each produce 4 time-series. Safe for Prometheus.
Violation counter cardinality: tier (3) × codec_id (~9) × media_type (4) × verdict (3) = ~324 max combinations. In practice, most participants use only 1-2 codecs, so actual cardinality is much lower.

Reviewer checklist (filled in by reviewer)

Code matches PRD intent
Verification output is real
No backward-incompat surprises
Tests cover the new behavior
Approved

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