Prometheus

It covers the essentials: instant and range vector selectors, rate calculations, aggregations, and common patterns like error rates and resource saturation queries. You also get guidance on alerting with both Alertmanager and Grafana, recording rules for pre-computation, and the newer Metrics Drilldown feature for queryless exploration. The PromQL quick reference alone is worth having on hand when you're writing queries or debugging dashboards. Most useful when you're setting up monitoring, writing alerts, or need to remember the difference between rate and irate.