title: Prometheus Alertmanager Silencing alerts via UI does not prevent first occurence of alert to trigger
body: <p>Can the ‘Silences’ feature on the Alertmanager UI be used to silence new alerts before they are sent to a receiver? This would be useful while performing an upgrade in an environment. </p>
  
  <p>My alerts contain a label 'env' (to identify a given environment) and I can add a silence rule via the UI.  The problem is that it will only silence AFTER the first occurrence of an alert in the 'env'/environment. In the meantime, an alert has been sent to the Alertmanager receiver.</p>
  
  <p>I have also tried changing values in the alertmanager.conf for 
  <code>group_interval</code> from 5m to 10s, and <code>group_wait</code> from 30s to 60s. </p>
  
  <p>Alternatively, I could update the receiver when there is a match for the 'env' label...but I would prefer to use the 'Silences' feature (either in UI or API - have not tried using the API to do silence).</p>
  
  <p>Any inputs would be appreciated.</p>
  
  <p>(Using version 0.14.0 of Alertmanager)</p>