New Relic
New Relic is an enterprise grade observability solution in a SaaS package.
They offer a uniform approach to dealing with metrics, logs and events - including a basic but working alert management feature.
The service is easy to get started with and has a generous free tier that works well for testing Butler alerts.
New Relic is a good choice for both metrics storage and alerting as it handles both reload failure alerts generated by the Butler tool as well as operational metrics from Butler SOS.