Metrics: numeric measurements over time. Types: counters (requests), gauges (memory), histograms (latency). Components: collection (Prometheus), storage (time-series DB), visualization (Grafana).
Pull vs push: Prometheus pulls from targets, StatsD/Telegraf push. Retention tiers: s resolution for hour, m for week, h for year. Aggregation reduces storage. Alert on thresholds and anomalies.