title: Unable to track pwa add to home screen events
body: <p>I have create a service worker file and manifeast  file and register service worker file in my index.html file. I want to track that how many users can see "Add to home screen button" and how many users have click "Add to home screen" . MY application has meet all the criteria of pwa. I have used "beforeinstallprompt" event in service file but its not fire when "Add to home screen" shows.</p>
  
  <p>My service worker code are given below.</p>
  
  <pre><code>var doCache = true;
  var version = 481;
  // Name our cache
  var CACHE_NAME = 'health-cache-v='+version;
  var filesToCache = [
  'index.html'
  ];
  
  let deferredPrompt;
  
  self.addEventListener('beforeinstallprompt', (event) =&gt; { 
   onsole.log('add to home screen popup show'); 
  event.userChoice.then(choiceResult =&gt; {
  if (choiceResult.outcome === 'accepted') {
      console.log('User accepted the add to home screen popup'); 
    } else {
      console.log('User dismissed the add to home screen popup');
  
    }     
  });
  });
  
  
  // Delete old caches that are not our current one!
  self.addEventListener("activate", event =&gt; {
    const cacheWhitelist = [CACHE_NAME];
  event.waitUntil(
    caches.keys()
      .then(keyList =&gt;
        Promise.all(keyList.map(key =&gt; {
        if (!cacheWhitelist.includes(key)) {
          console.log('Deleting cache: ' + key)
          return caches.delete(key);
        }
      }))
     )
    );
   });
  
  
   self.addEventListener('install', function(event) {
   if (doCache) {
     event.waitUntil(
      caches.open(CACHE_NAME)
        .then(function(cache) {       
          cache.addAll(filesToCache);         
            console.log('cached');
      })
     );
    }
   });
  
   self.addEventListener('fetch', function(event) { 
    if (doCache) {         
      event.respondWith(
        caches.match(event.request).then(function(response) {
            return response || fetch(event.request);
        })
      );
     }
    }); 
  </code></pre>