title: Grafana query to get unique tag values
body: <p>I want to get unique values for specific tag of some metric.
  For example if I have metric 'metric_name' has tags 'tag_name1' and 'tag_name2':</p>
  
  <pre><code>metric_name{tag_name1='a',tag_name2='b'}
  metric_name{tag_name1='c',tag_name2='d'}
  metric_name{tag_name1='e',tag_name2='f'}
  </code></pre>
  
  <p>I want to get unique values of 'tag_name1' tag: a,c,e</p>
  
  <p>Kind of like:</p>
  
  <pre><code>select distinct tag_name1 from metric_name
  </code></pre>