title: Prometheus/PromQL subtract two gauge metrics
body: <p>I have this gauge metric "metric_awesome" from two different instances.
  What i want to do, is subtract instance one from instance two like so</p>
  
  <pre><code>metric_awesome{instance="one"} - metric_awesome{instance="two"}
  </code></pre>
  
  <p>Unfortunately the result set is empty. Has anyone experienced this? </p>