title: Using alert annotations in an alertmanager receiver
body: <p>I've got an alert configured like this:</p>
  
  <pre><code>ALERT InstanceDown
  IF up == 0
  FOR 30s
  ANNOTATIONS {
      summary = "Server {{ $labels.Server }} is down.",
      description = "{{ $labels.Server }} ({{ $labels.job }}) is down for more than 30 seconds."
  }
  </code></pre>
  
  <p>The slack receiver looks like this:</p>
  
  <pre><code>receivers:
  - name: general_receiver
    slack_configs:
    - api_url: https://hooks.slack.com/services/token
      channel: "#telemetry"
      title: "My summary"
      text: "My description"
  </code></pre>
  
  <p>Is it possible to use the annotations in my receiver? <a href="https://github.com/prometheus/alertmanager/issues/250#issuecomment-239305395" rel="noreferrer">This github comment</a> indicates it is but I haven't been able to get anything to work from it.</p>