# 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

```bash
$ 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
```

```bash
$ 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