## TL;DR

Prometheus is the de facto open-source monitoring and alerting system, graduated from CNCF. It scrapes metrics from instrumented targets via HTTP, stores time-series data locally, and provides a powerful query language (PromQL). Paired with Grafana for visualization and Alertmanager for notifications.

## Core Explanation

Pull-based model: Prometheus server scrapes targets at intervals (15s default). Instrumentation: client libraries expose /metrics endpoint. PromQL: `rate(http_requests_total[5m])`, `histogram_quantile(0.99, ...)`. Service discovery auto-detects targets (Kubernetes, Consul, EC2). Limitations: single-node, not suitable for long-term storage (use Thanos/Cortex for that).

## Further Reading

- [Prometheus Documentation](https://prometheus.io/docs/)