Grafana Data Center selector menu shouldn't just always default to the first item in the menu

[roybatty2019]

Is your feature request related to a problem? Please describe.
We have several Data Centers (DCs) configured. When opening a new dashboard, the Grafana UI always defaults to displaying information for only the first DC in the menu. But that particular DC is of no special significance to us, indeed just showing those values is somewhat misleading, every time I open a new dashboard my first thought is: Where has the other 10 TB gone!?!

Describe the solution you'd like
It should be possible to set the default DC used to be any or all of those configured.
For us it would be especially useful to be able to set the default to "all".
(I suppose from a UI design viewpoint this should probably be a per user preference option. But that is not relevant for us.)

Describe alternatives you've considered
I understand it might be possible to achieve this by customising each dashboard individually, but that would be considerable effort.

Additional context
See also Issue: #3384 Dashboard variables: set default value on import.