mesure abandon avant GA.js

// idée originale : https://developers.google.com/web/fundamentals/performance/user-centric-performance-metrics#load_abandonment
window.__trackAbandons = () => {
  // Remove the listener so it only runs once.
  document.removeEventListener('visibilitychange', window.__trackAbandons);
  const ANALYTICS_URL = 'https://www.google-analytics.com/collect';
  const GA_COOKIE = document.cookie.replace(
    /(?:(?:^|.*;)\s*_ga\s*\=\s*(?:\w+\.\d\.)([^;]*).*$)|^.*$/, '$1');
  const TRACKING_ID = 'UA-XXXXX-Y';
  const CLIENT_ID =  GA_COOKIE || (Math.random() * Math.pow(2, 52));

  // Send the data to Google Analytics via the Measurement Protocol.
  navigator.sendBeacon && navigator.sendBeacon(ANALYTICS_URL, [
    'v=1', 't=event', 'ec=Load', 'ea=abandon', 'ni=1',
    'dl=' + encodeURIComponent(location.href),
    'dt=' + encodeURIComponent(document.title),
    'tid=' + TRACKING_ID,
    'cid=' + CLIENT_ID,
    'ev=' + Math.round(performance.now()),
  ].join('&'));
  document.removeEventListener('visibilitychange', window.__trackAbandons);
};
document.addEventListener('visibilitychange', window.__trackAbandons);