Why is my prometheus data source not listed for ad-hoc variables?

I have a dashboard which is working against a Prometheus datasource (both Grafana and Prometheus were installed from latest version in the last month).

When I try to add an adhoc variable, the datasource drop-down looks weird:

If I change the type to Query then my Prometheus datasource appears again. What is going on?

I’m also pretty unclear how to use adhoc properly. Our metrics server is monitoring several systems where each server has labels Customer and System. I have variables for each of these and then all my panel queries include {Customer='$Customer',System='$System'} so I might select from my drop-downs ‘Microsoft’ and ‘Production’.

My understanding is I can set up an adhoc variable to automatically add this to my queries?