metrics-expositionlisted

# Metrics exposition: bounded cardinality, and truncation you can see **Read [`references/METRICS-EXPOSITION.md`](references/METRICS-EXPOSITION.md) before applying any of this.** That file is the standard; everything below it is a summary to help you decide whether this skill applies and to check your work afterwards. Reference-architecture principles: P15. ## What this standard covers - Every label is a cardinality decision - High-cardinality dimensions are opt-in and capped - Export the fact that you truncated - Emit sums and counts, not averages - The endpoint is a surface; it can be closed - Dashboards are provisioned, not clicked ## Failure modes | Symptom | Cause | |---|---| | Prometheus memory climbs steadily and never recovers | A label carrying ids from your data — session, user, request, a path with an id in it. Each distinct value is a permanent series (§1) | | Monitoring costs more than the service it monitors | The same, discovered on a bill rather than in a graph | | A panel's numbers are confidently wrong, and nothing looks broken | The exporter truncated and never said so, so the dashboard is rendering a capped view as if it were complete (§3) | | A rollup across instances disagrees with the per-instance panels | Pre-averaged gauges being averaged again — an average of averages (§4) | | Cardinality is fine in staging and explodes in production | The high-cardinality view is default-on; staging simply never had enough distinct values to show it (§2) | | A