[monitoring] Review tweaks to resource usage dashboards

[jnywong]

Context

A few review requests have come in the https://github.com/jupyterhub/grafana-dashboards repo recently. These are small enhancements to the visualisations of some of the resource usage dashboard panels.

What we need to do

Review the following

• jupyterhub dashboard: transition two panels to heatmaps, and tweak existing heatmaps jupyterhub/grafana-dashboards#161
• Change CPU panels' units from percent to sishort (20% -> 200m) jupyterhub/grafana-dashboards#163
• Make non-running pods panels consistent and style them intuitively jupyterhub/grafana-dashboards#164
• Provide username_safe label for kubespawners modern escaping jupyterhub-groups-exporter#41
• Username mapping for both of kubespawner's escaping schemes jupyterhub/grafana-dashboards#168

Definition of Done

• The feature/service is technically complete
• The feature/service follows our definition of minimally documented

[jnywong]

This is not initiative-driven but tangentially related to #6321 which is in flight.

[jnywong]

I've merged one and given review comments to the others, so now we are pending responses 🙂

[jnywong]

All of the PRs are approved so closing as complete 🚀