P1: Expose Prometheus metrics at /metrics (#173)#191
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There was no metrics endpoint — only optional Jaeger tracing — so request rate,
errors, latency, and process health were invisible in production.
Add a prom-client registry served at `/metrics`:
- RED HTTP metrics via a Yoga plugin: `http_requests_total{method,route,status}`,
`http_request_duration_seconds` histogram, and `http_requests_in_flight`.
- Standard Node process metrics (CPU, memory, event loop, GC).
- Route labels are normalised to a known set (`other` otherwise) to bound
cardinality; the `/metrics` scrape is not self-counted.
DB pool-saturation gauges are intentionally out of scope here: postgres.js
exposes no pool-introspection API, so that needs a query-instrumentation pass —
tracked as a follow-up.
Unit tests cover the exposition format, per-route/status counting, and that the
scrape endpoint is excluded.
Refs #173.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QSuak9smCHbp4N17xjjLF6
dkijania
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production-readiness
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What & why
Part of the production-readiness epic (#163). Refs #173.
There was no metrics endpoint — only optional Jaeger tracing — so request rate, errors, latency, and process health were invisible in production.
Changes
/metricsendpoint (prom-client) via a Yoga plugin.http_requests_total{method,route,status},http_request_duration_secondshistogram,http_requests_in_flightgauge./,/healthcheck,/readiness,/metrics; anything else →other) to bound label cardinality; the/metricsscrape is not self-counted.New dependency:
prom-client.On DB pool saturation
The issue also lists DB pool-saturation gauges.
postgres.jsexposes no pool-introspection API, so those need a query-instrumentation pass rather than a simple read of pool state. I scoped that out of this PR and left #173 open for it (this PR isRefs, notCloses) — happy to follow up with an instrumentation approach if you want it.Testing
npm run build— cleannpm run test:unit— all pass; tests cover the exposition format, per-route/status counting (asserts a request count of 2), and exclusion of the scrape endpointnpm run lint/npx prettier --debug-check .— clean🤖 Generated with Claude Code