Either install the grafana/prometheus stack yourself (native or via docker) or use the hosted central server for monitoring
An example minetest.conf with the pushgateway at port 9091:
secure.http_mods = monitoring
monitoring.prometheus_push_url = http://127.0.0.1:9091/metrics/job/minetest/instance/my_server
Enables more metric at a slightly increased cpu-usage:
monitoring.verbose = true