Write unified tutorial with prometheus

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https://prometheus.io/docs/practices/alerting/#metamonitoring

Metamonitoring

It is important to have confidence that monitoring is working. Accordingly, have alerts to ensure that Prometheus servers, Alertmanagers, PushGateways, and other monitoring infrastructure are available and running correctly.

As always, if it is possible to alert on symptoms rather than causes, this helps to reduce noise. For example, a blackbox test that alerts are getting from PushGateway to Prometheus to Alertmanager to email is better than individual alerts on each.

Supplementing the whitebox monitoring of Prometheus with external blackbox monitoring can catch problems that are otherwise invisible, and also serves as a fallback in case internal systems completely fail.