Add error budget to stage group dashboards

Bob Van Landuytrequested to merge
bvl-error-budget-on-dashboard into master

This is the first iteration that shows the error budget on a stage group dashboard. It replaces the old SLI panels of the stage group dashboard. Since those would only show web requests, while the error budgets will eventually aggregate all components.

The error budget calculation always takes 30d, not the range specified in Grafana.

Part of gitlab-com/gl-infra/scalability#945

Example dashboards:

Edited by Bob Van Landuyt

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