Prometheus exporter
An adapter that exposes metrics from a system in Prometheus text format (/metrics endpoint). Prometheus scrapes these periodically.
node_exporter (host metrics — CPU, RAM, disk, network) is the most used. Thousands of official + community exporters: postgres_exporter, mysqld_exporter, blackbox_exporter (ping/HTTP checks), nginx_exporter, smartmontools_exporter (disk health). Apps can directly expose natively (Java prometheus_client, Go prometheus/client_golang). OpenTelemetry now competes — push instead of pull, more signals.