Add metrics endpoint#4
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Synthesize a Prometheus /metrics endpoint from the per-request data we
already receive, plus live worker-pool state. tsheadroom calls headroom's
bare compress() in isolated workers, so headroom's own aggregate metrics
(proxy/client Prometheus, OTel) are never populated by us; instead we
accumulate our own lifetime counters in Go.
Two families are emitted:
- headroom_*: reuse the names headroom's proxy emits for the subset we
can populate faithfully (requests, tokens in/saved, our processing
overhead, per-provider/model, inbound HTTP), so existing headroom
dashboards can be repointed at this endpoint.
- tsheadroom_*: native metrics with no headroom analog (pool
saturation gauges, per-outcome counts, cold starts, tokens_after).
Response-dependent proxy metrics (completion tokens, TTFB, cache
read/write/bust, per-transform timing, waste signals) have no source on
a pre_request hook and are deliberately omitted rather than reported as
misleading zeros.
Also parse aperture's always-present metadata.{provider,model} (sent
regardless of the hook's send config) and prefer metadata.model as the
authoritative model for compress() and the metric labels, falling back
to the request body's model field.
The Metrics type is nil-safe so the handler tests construct a Handler
without one. The pool exposes a busy-slot atomic for the live gauges.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Luke Kosewski <lkosewsk@tailscale.com>
Add a Metrics section to the README: the curl invocation, tables for the headroom_* (repoint-compatible) and tsheadroom_* (native) families, the rationale for the omitted proxy-only metrics, and an access note. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com> Signed-off-by: Luke Kosewski <lkosewsk@tailscale.com>
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Simulate headroom's own proxy metrics endpoint.
Add a few other fun metrics that at tsheadroom_ metrics only.