blog: Cardinality Limits in OpenTelemetry — When Your Metrics Lie, and What to Do About It

### Blog Post Title Cardinality Limits in OpenTelemetry — When Your Metrics Lie, and What to Do About It ### Blog Post Outline A practitioner-focused walkthrough of the OpenTelemetry metrics SDK cardinality cap: what it is, why the spec mandates it, the `otel.metric.overflow=true` marker, and the surprising query-time consequence — filtered queries silently undercount while totals stay correct. Closes with two concrete actions: a fleet-wide audit query and a standing alert for new overflow series. Companion to the KubeCon EU 2026 Observability Day lightning talk "Metrics That Lie: OTel's Cardinality Capping Trap". https://youtu.be/QTeA16I_hME?si=x1ylhiztDAec0Un9 ### Technologies Used OpenTelemetry Metrics SDK, OpenTelemetry Specification, Prometheus / PromQL, OTLP. ### Related Special Interest Groups (SIGs) SIG Specification, SIG Metrics ### Sponsoring SIG SIG Specification ### Sponsor Name @pellared