Mathias e49b36e463 feat(ingestion): expose Prometheus /metrics for brain query latency ...

Closes infra#50.

Adds an internal/metrics package with a hand-rolled Prometheus
exposition layer (stdlib + sync/atomic only — no new dep) and wraps the
HTTP mux with a timing middleware. Every request emits one observation
on the `brain_query_duration_seconds` histogram labeled by
`path` (request Pattern, low cardinality) and `status` (2xx/3xx/4xx/5xx).

Dependency choice: hand-rolled rather than github.com/prometheus/client_golang
because the surface needed is small (one histogram + bucket constants)
and the repo CLAUDE.md keeps deps stdlib + jwx + testify only. ~150 LOC
of code + tests is cheaper than the chart of transitive prometheus deps.

Endpoints:
- GET /metrics  — OpenMetrics text exposition, no auth (cluster-internal)

Wire format pinned by tests in internal/metrics/metrics_test.go. The
ServiceMonitor that drives the kube-prometheus-stack scrape lives in
infra/k3s/apps/supervisor/ (separate commit on mathias/infra).

After this image deploys, the canary alert from
docs/superpowers/specs/2026-05-homelab-architecture-review.md becomes
wireable:

  histogram_quantile(0.95,
    sum(rate(brain_query_duration_seconds_bucket[5m])) by (le))
    > 1.5 * histogram_quantile(0.95,
        sum(rate(brain_query_duration_seconds_bucket[5m] offset 7d)) by (le))

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

2026-05-22 07:13:05 +02:00

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