Prometheus, Grafana, and Alert Manager are a good stack to monitor the validator node performance. A comprehensive step-by-step setup process is documented in an official wiki.
A fellow validator uses the same monitoring stack and publish a Medium blogpost. It is quite similar to the official guide, but does a better job in explaining the conceptual framework and how different pieces fit together.
I also recommend a log monitoring system to gain more visibility into the node performance. Logs are especially helpful when you go trouble-shooting. I use Promtail to collect logs on validator server and Loki to receive these logs and feed into Grafana on the monitoring server.