The math

Cardinality = product of unique label values across all labels. A metric with (region=10) × (env=3) × (service=50) × (user_id=10M) = 15 billion unique series. Each series consumes memory + storage. Costs scale linearly with cardinality, not request volume.

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Label hygiene rules

Never label with high-cardinality user data: user_id, email, request_id, full URLs. Bucketize what you can (latency_bucket=fast/slow). Use enums for known-small sets (status_code, region). Logs/traces hold the high-cardinality data, not metrics.

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Offending patterns

'Track per-user error rate' as a metric — wrong; do it as a log query. 'Track per-customer latency' — use exemplars on a single latency histogram. 'Track per-endpoint with /users/{id} unhashed' — route templates, not URLs.