Add Monitoring/Alerts Section for Gravity 6.X+ discuss setup of Prometheus

[aelkugia]

Current Monitoring/Alerts section pertains to Gravity 5.5 and earlier

This task covers extending the training section to involve Gravity 6.X+

Also a new section should be added to the Monitoring/Alerts discuss setup of Prometheus to scrape metrics from a custom pod/deployment. Some customers have been looking at running a simple pod and would like to see the metrics scraped from it to the default monitoring gravity UI console. In discussion with Roman, the suggested approached would be:

Since we’re just deploying a Prometheus Operator (https://github.com/coreos/prometheus-operator) to configure it to scrape metrics from a given service, you can either create additional scrape config (https://github.com/coreos/prometheus-operator/blob/master/Documentation/additional-scrape-config.md) or declare it via a ServiceMonitor resource (it’s described in https://github.com/coreos/prometheus-operator/blob/master/Documentation/user-guides/getting-started.md).

Then, you will need to create a Grafana dashboard and how to plug it into our UI is described in https://gravitational.com/gravity/docs/monitoring/#grafana-integration.