No, you don't generally end up with agents or exporters everywhere. You might have node_exporter running on each host to collect system metrics (load, iops, etc

No, you don't generally end up with agents or exporters everywhere. You might have node_exporter running on each host to collect system metrics (load, iops, etc.) but those are exceptions. The rule is that the software under instrumentation is mounting /metrics endpoints and exposing Prometheus metrics to the Prometheus server directly. Unless you take the presence of node_exporter to invalidate the premise (which would be stupid) a system that's exclusively pull-based is entirely practical.