Create a few Grafana dashboards that we can use for reporting

[GeorgianaElena]

Context

A quick update on this one as we discussed this a bit in the sustainability team meeting. Another useful aspect of reporting and monitoring is if we can quickly answer questions that demonstrate usage and impact for our hubs. So things like:

• How many unique users logged in to (all, or a specific) 2i2c hub today/this week/this month?
• How much total time was spent in interactive sessions for all users?
• How long is the average user session? How many times a week do they log in?
• Any kind of usage of specific services like Dask Gateway

I think most of these should be doable with the prometheus logs that each hub is generating, just listing here so we don't lose track of it!

Originally posted by @choldgraf in #328 (comment)

Proposal

We should try to add as much as these in the upstream repo https://github.com/jupyterhub/grafana-dashboards
Not sure where to keep the ones that are more 2i2c specific though?

Updates and actions

• Support having dashboards aggregating info from multiple datasources (clusters) -> Add support for global dashboards jupyterhub/grafana-dashboards#39

[GeorgianaElena]

Closing as pretty old and we are tracking new grafana dashboards into their own issues anyway.