Create a new custom dashboard with a single chart

In the Monitor Sandbox project we currently have a single_stats_custom_dashboard.yml that shows a single-stat metric, but it would be nice to have another custom dashboard with area-chart or similar so we can see if there are rendering errors like in gitlab-org/gitlab#121932 (closed).

We could probably use one of our existing common metrics like "core usage", just in a custom dashboard.

/cc @dbodicherla