prometheus alerting is not user-friendly, you can read more in official documentation [url]https://prometheus.io/docs/alerting/overview/[/url]
Alerts in Grafana simpler (if you have graph for needed metric):
[url]https://www.percona.com/blog/2017/01/23/mysql-and-mongodb-alerting-with-pmm-and-grafana/[/url]
[url]https://www.percona.com/blog/2017/02/02/pmm-alerting-with-grafana-working-with-templated-dashboards/[/url]