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Section 5 · Correlate signals

Tracking request latency without a per-user series explosion

Estimated time: 1 min

A working example

Suppose you track request latency and want to find slow requests for specific users.

  • Instead of http_request_duration_seconds{user_id="u-48221", ...}, which creates a series per user per label combination,
  • keep the metric as http_request_duration_seconds{service="checkout", env="prod"},
  • and log each request with user_id and request_id attached as structured metadata.

When latency spikes, the metric tells you where and when. Then you query the logs for that service and window, filtering by structured metadata: {app="checkout"} | user_id="u-48221". You keep full debugging detail without paying for it on every series.