What monitoring tools have you used, particularly open‑source ones?

I have used several open‑source monitoring solutions. Prometheus is my primary metric collector; it scrapes metrics from services via HTTP endpoints and stores them in a time‑series database. Grafana is used for visualizing Prometheus data, creating dashboards, and setting alerts. For log aggregation, I rely on the ELK stack (Elasticsearch, Logstash, Kibana) or its lightweight alternative, Loki. I’ve also configured Alertmanager to route alerts from Prometheus to Slack or email. For infrastructure monitoring, I’ve used Zabbix and Nagios for server health checks and network monitoring. These tools together provide end‑to‑end visibility into application performance, resource utilization, and operational health.