docs(adr): ADR-030 observability — slog + Prometheus metrics (issue #15)

DECISIONS.md

@@ -1162,6 +1162,63 @@ re-login now rare (30-day idle or explicit logout), it matters far less.
 
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+## ADR-030 — Observability: slog timing + Prometheus metrics (AI-focused)
+
+**Status:** Proposed (2026-06-11). Issue #15. **Draft for review — no code yet.**
+
+**Context / requirements.** Nothing measures the activities that drive Tapir's performance and
+UX, and the Stage-0 eval gate (ADR-016) needs a *performance* dimension to sit beside the
+return-usage one. We need timing for: caption fetches (the scarce op), summarization (which model
+won, how long, fallbacks), Q&A latency, LLM token spend, and basic session/usage (request rate,
+latency by route, logins). Requirements:
+- R1: structured `slog` timing at each AI call site (human-readable, already the logging stack).
+- R2: Prometheus metrics for the same, scrapeable by the cluster's prometheus-operator.
+- R3: **AI metrics are the priority** — summarize latency by `model`/`outcome`/`fallback`,
+  caption-fetch latency by `outcome`, chat latency by `model`, and LLM `tokens` by model+kind.
+- R4: HTTP/session metrics via middleware — request count + latency by route, logins.
+- R5: bounded label cardinality (no per-user, no raw-path labels).
+- R6: `/metrics` must NOT be publicly exposed.
+
+**Decision / architecture.**
+1. **New package `internal/metrics`** owns all Prometheus collectors + a typed API
+   (`ObserveSummarize`, `ObserveCaptionFetch`, `ObserveChat`, `RecordTokens`, `IncLogin`,
+   `HTTPMiddleware`, `Handler`). Adapters call this API; they never import prometheus types.
+2. **New dependency `github.com/prometheus/client_golang`.** Justification: it is *the* standard
+   Go Prometheus client and the cluster already runs prometheus-operator; hand-rolling exposition
+   is not worth it. (Needs the dep-justification note in the commit per repo rules.)
+3. **The copied `llm` package stays stdlib-only (ADR-004).** It must not import `internal/metrics`.
+   Token usage is surfaced via an **optional callback** `llm.WithUsageHook(func(model string, prompt, completion int))`
+   set at wiring time (`buildSummarizer`/`buildChat`) to `metrics.RecordTokens`; `llm.Client` only
+   gains parsing of the response `usage` block. Our own adapters (`summarizer`, `youtube`, `chat`)
+   may import `internal/metrics` directly.
+4. **HTTP middleware** reads `r.Pattern` AFTER routing (Go 1.22 sets it during ServeMux match), so
+   the `route` label is the bounded registered pattern (`GET /v/{videoId}`), satisfying R5;
+   unmatched → `other`.
+5. **Dedicated metrics port** (`TAPIR_METRICS_ADDR`, default `:9090`) served by a second
+   `http.Server` in `cmdServe`; `/metrics` is never on the public app mux (R6). A **PodMonitor**
+   in `mathias/infra` scrapes it; the deployment exposes the port.
+6. **slog** elapsed fields are emitted alongside each metric at the call sites (R1).
+
+**Hook points (where the instrumentation lands).**
+- `summarizer.Summarize` — per-endpoint timing + outcome (`success`/`parse_error`/`error`) + fallback flag.
+- `youtube.FetchTranscript` — fetch timing + outcome from `domain.Transcript.Source`.
+- `chat.Service` answer — timing by model.
+- `llm.Client.Complete` — parse `usage`, fire the usage hook.
+- `oidc.handleCallback` — `IncLogin`.
+- `cmdServe` — wrap `Router()` in `metrics.HTTPMiddleware`; start the metrics server.
+
+**Out of scope / later.** Persisting per-summary latency into Postgres for `tapir report`
+(derive UX latency — publish/discovery → summary — from existing timestamps first; only persist
+op-latency if the scrape proves insufficient). SPA view (#16) and visual refresh (#17).
+
+**Reversibility.** Additive: a new package + middleware + a metrics port. Removing the PodMonitor
+stops scraping; the app is unaffected. No schema change.
+
+**Next steps (gated):** on approval of this ADR → BDD scenarios (`docs/use-cases/observability.feature`
++ scenario-coverage map) → TDD → implement → SemVer + docs + PodMonitor.
+
+---
+
 ## Rejected alternatives
 
 Approaches considered during the 2026-06-02 planning + grill session and **deliberately not