Prometheus query to count unique label values

Question

I want to count number of unique label values. Kind of like

select count (distinct a) from hello_info

For example if my metric \'hello_info\'

Answer 1

Other example: If you want to count the number of apps deployed in a kubernetes cluster based on different values of a label( ex:app):

count(count(kube_pod_labels{app=~".*"}) by (app))