If the servers are not appearing in the host drop-down on the Grafana dashboard, ensure to check the following two scenarios:
Verify that the scrape targets (servers) are correctly configured in Prometheus
Check the scrape_configs section in the Prometheus configuration file to ensure that the correct endpoints, ports, and protocols are specified for each target. Ensure that the scrape targets are actively exposing metrics in the expected format.
Validate metrics availability
Verify that the targets for the scrape are actually exposing metrics for the Prometheus scraper. You can use tools like curl or Prometheus' expression browser to directly query the scrape target's metrics endpoint and verify if the expected metrics are returned.