title: Prometheus query to count unique label values
body: <p>I want to count number of unique label values. Kind of like </p>
  
  <pre><code>select count (distinct a) from hello_info
  </code></pre>
  
  <p>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". </p>
  
  <pre><code>hello_info(a="1", b="ddd")
  hello_info(a="2", b="eee")
  hello_info(a="1", b="fff")
  hello_info(a="3", b="ggg")
  </code></pre>