Replace koa-prometheus-exporter with prom-client#79
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The @echo-health/koa-prometheus-exporter wrapper hard-pinned prom-client ^10.2.3, which only emits the mean event-loop lag gauge (nodejs_eventloop_lag_seconds) and routed GC through the legacy prometheus-gc-stats package. As a result the perf_hooks-based collectors were missing: event-loop lag percentiles (nodejs_eventloop_lag_p50/p90/p99 _seconds) and the nodejs_gc_duration_seconds histogram, both added in prom-client v12. Drop the wrapper (and its prometheus-gc-stats dependency) and depend on prom-client ^15.1.3 directly. collectDefaultMetrics() is called once at module load, and useObservability() serves /metrics from the default registry. The existing http_requests_total counter (labels method,status) is preserved unchanged; nothing in prom-client's default metrics replaces it. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_015kkNPwFkKLCTmbRYerVwSy
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Summary
Replaces the
@echo-health/koa-prometheus-exporterdependency with the standardprom-clientlibrary, implementing a custom metrics endpoint middleware instead of relying on the wrapper package.Changes
@echo-health/koa-prometheus-exporter, addedprom-client(^15.1.3)promClient.collectDefaultMetrics()at module load to collect Node.js runtime metrics (event-loop lag, GC duration, heap usage, etc.)prometheus.middleware({})with a custommetricsEndpointmiddleware that:/metricsContent-Typeheader from the prom-client registrypromClient.register.metrics()const promClient = prometheus.clientassignment and now useprom-clientdirectlyImplementation Details
The custom middleware approach provides more explicit control over the metrics endpoint while maintaining the same functionality. The
collectDefaultMetrics()call is placed at module load inapp-common.js(which is cached) so that in production, where www and download run as separate processes, each process gets its own collector instance.https://claude.ai/code/session_015kkNPwFkKLCTmbRYerVwSy