Netdata

Netdata installs in one command and immediately starts charting thousands of per-second metrics — CPU, disks, containers, databases — with automatic detection of what is running on the box. The out-of-the-box dashboard is unmatched for "what is happening right now".

The agent is efficient, but per-second granularity everywhere generates a lot of data; long-term retention and multi-node views push you toward its cloud offering or exporting into Prometheus. The open-source agent remains fully usable standalone.

Best as an instant, detailed magnifying glass on individual machines, complementing rather than replacing a Prometheus setup.