Phase H ("retire the Python proxy") needs cache-hit-rate parity
between the Rust and Python proxies during canary. This PR lands
the per-invocation RTK metrics and the proxy-side observability
surface that the canary gate depends on.
Rust observability:
- `proxy_cache_hit_rate_per_session{provider}` — histogram, emitted
per session at SSE state-machine close (Anthropic message_delta,
OpenAI Chat final usage chunk, OpenAI Responses response.completed).
The Phase H canary gate metric.
- `proxy_compression_ratio_by_strategy{strategy, content_type}` —
histogram; one sample per shrunk block.
- `proxy_compression_rejected_by_token_check_total{strategy}` —
counter for tokenizer-validated rejections.
- `proxy_passthrough_bytes_modified_total{path}` — counter (must
stay 0 outside compression hot path; alarmable via PromQL rate).
- `proxy_rate_limit_remaining_{requests,tokens,input_tokens,output_tokens}{provider}` —
gauges populated from anthropic-ratelimit-* / x-ratelimit-* headers.
- `proxy_service_tier_count_total{tier}` and
`proxy_response_status_count_total{status}` — counters for
Responses-API outcome telemetry.
- `proxy_image_generation_call_log_redacted_total` — counter.
- `wrap_rtk_invocations_total{tool}` and
`wrap_rtk_tokens_saved_per_session` — RTK metrics exposed via
the proxy's /metrics scrape so wrap-side tail can increment
through one observability surface.
All metric names and label keys live in a single
`observability/metric_names.rs` constants module per realignment
build-constraint "configurable". Bounded label vocabularies
(service_tier, response_status, provider) are defined alongside.
Python (P4-45):
- `headroom/proxy/request_logger.py` — base64-image payloads in
request/response logs over 1024 bytes are replaced with
`<image:base64-redacted bytes=N>` placeholders. Walks Anthropic
source.data and OpenAI data URLs. No regexes — substring +
density heuristic.
Tests:
- `crates/headroom-proxy/tests/integration_metrics.rs` — 6 tests
covering cache-hit-rate, compression-ratio, passthrough-bytes,
service-tier, response-status, and rate-limit-snapshot.
- `tests/test_image_log_redaction.py` — 13 tests for the Python
redaction helper.
- Existing tests: 1100+ Rust + 76 Python regression checks green.
Docs:
- `docs/observability.md` — metric catalogue + PromQL queries.
- `docs/rtk-architecture.md` — locks the wrap-CLI-only decision so
future contributors don't relitigate proxy-side RTK.
No silent fallbacks: zero-denominator cache-hit-rate logs and
skips rather than synthesising 0.0. Unparseable rate-limit headers
stay None rather than coerced to 0. Missing upstream JSON fields
log + skip emit rather than fabricating data.
