Metrics
Numerical time-series. Aggregatable. Cheap. Prometheus + Grafana + StatsD. Golden signals (latency, traffic, errors, saturation).
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Logs
Structured events. Detailed context. Higher volume. Loki, Elasticsearch, CloudWatch, Datadog Logs. Search + alert.
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Traces
Request path across services. OpenTelemetry standard. Jaeger, Tempo, X-Ray. Distributed root-cause.