High-cardinality metrics just got safer with OpenTelemetry’s cardinality capping—SDKs won’t get OOM-killed anymore—but there’s a dangerous catch. When cardinality limits are hit, new attribute sets are folded into a special overflow attribute. The problem? Metric backends like Prometheus don’t treat this overflow attribute as special—it’s just another label—so queries can silently return incorrect results.
Imagine querying error counts for /endpoint1. Once the cap is hit, new errors for that endpoint get folded into overflow. A query like errors WHERE url="/endpoint1" might underreport errors—masking real issues.
This lightning talk explains how OTel’s cardinality capping works, how it can create hidden observability traps, how to detect when your queries may be incorrect, and strategies to avoid falling into this subtle but impactful pitfall.

The spec for cardinality capping is already stable, and support is rolling out across SDKs—now’s the time to understand its impact and prepare.