title: Alert if data in certain labels is missing in Prometheus
body: <p>I am sending data related to two disks to prometheus. I want to alert if metrics of one disk stops sending metrics. Say I have diskA and diskB and I am collecting disk_up metric. Now diskB failed. In prometheus</p>
  
  <p><code>disk_up{disk="diskA"}</code> will have data and 
  <code>disk_up{disk="diskB"}</code> will be missing</p>
  
  <p><code>absent(disk_up)</code> will be 0 since disk_up have diskA's data. 
  <code>absent(disk_up{disk="diskB"})</code> will serve the purpose. But I don't want to hardcode the disk names.</p>
  
  <p>Can I know which is the better way to setup an alert for this scenario.</p>