title: How can I get a reliable alert via Stackdriver when there are no clients pulling from a Pub/Sub subscription?
body: <p>I currently have some alerts set up to report when <code>subscription/pull_request_count</code> is 0. However, in a <a href="https://stackoverflow.com/questions/57486758/why-are-my-stackdriver-google-pub-sub-subscription-pull-request-count-metric-ale">similar question about that metric</a>, I found that metrics and alerting break once there is no data, which I believe happens when there are no subscriptions.</p>
  
  <p>My intent is to figure out if my servers have stopped pulling messages. There are 2 scenarios I have in mind where the details are important.</p>
  
  <ol>
  <li>Even if there are no messages being published, I want to know if I'm no longer pulling from a subscription to make sure things are working properly.</li>
  <li>In the event that a ton of unacknowledged messages are queued up just because I pulled them but didn't ack them (e.g. a partner API was down), I don't want this alert to be triggered</li>
  </ol>
  
  <p>Besides using <code>subscription/pull_request_count</code> as a condition, which won't work when no data is coming in (at least after a while), how can I set up an alert that notifies me that there no clients pulling from a Pub/Sub subscription?</p>