`NewLLMMetricsCapture`/`ResponseWriterWrapper` had **zero call sites**, so `llm_ttft_seconds`, `llm_itl_seconds` and `llm_tokens_total` were exported as HELP/TYPE stubs with no samples (verified against the live `/metrics`).
These are the only per-request LLM latency series covering the Ollama models (`ornith:35b`, `qwen2.5:3b-instruct`), which expose no `/metrics` of their own — so those models had zero latency visibility.
## Change
Wire the wrapper around the response writer in `ServeHTTP` for model-routed requests, and delete the unused `io.WriteCloser` duplicate that made the gap hard to spot (two near-identical implementations, neither constructed).
## Correctness details
- **Only SSE responses are measured.** A non-streaming completion is written in one `Write`, which would otherwise be recorded as a TTFT sample equal to the whole request latency, with no ITL samples — plausible-looking but meaningless numbers.
- **`requestStart` is captured at the top of `ServeHTTP`** so TTFT measures from request acceptance, not from when the upstream response is copied.
- **Token count skips the `data: [DONE]` terminator.**
- **`Unwrap()`** keeps `http.ResponseController` (Flush/Hijack/deadlines) working through the wrapper, so SSE unbuffering (#33) is unaffected.
## Tests
| Test | Guards |
|---|---|
| `TestResponseWriterWrapperRecordsSSEMetrics` | TTFT=1, ITL>0, tokens=2 (terminator excluded) |
| `TestResponseWriterWrapperIgnoresNonStreaming` | JSON response records nothing |
| `TestResponseWriterWrapperPreservesStreaming` | bytes + Flush pass through |
| `TestHandlerRecordsLLMMetricsForStreaming` | end-to-end through `ServeHTTP` |
The last one fails if the wrapper is ever wired nowhere again — verified by disabling the wiring locally and watching it fail.
## Verification needed post-merge
Confirm samples appear:
```
curl -s https://api.riotpiao.com/metrics | grep -E "llm_(ttft|itl)_seconds_count|llm_tokens_total"
```
