I think you are right that the "really traditional" monitoring system like Nagios or Munin only work through agents and plugins. Some of the hosted systems have

I think you are right that the "really traditional" monitoring system like Nagios or Munin only work through agents and plugins. Some of the hosted systems have centralized APIs. But, to my knowledge, they all use the metric concept ((name, value) pairs) and it's why I called it "traditional".