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' has labels a and b. I want to count number of unique a's. Here the count would be 3 for a = "1", "2", "3".

hello_info(a="1", b="ddd")
hello_info(a="2", b="eee")
hello_info(a="1", b="fff")
hello_info(a="3", b="ggg")

Answer 1

count(count by (a) (hello_info))

First you want an aggregator with a result per value of a, and then you can count them.