Add Google Analytics.

Author: schungx

index.html

@@ -128,4 +128,20 @@ <h3 class="space-lg">IDE integration</h3>
     </div>
 </body>
 
+<!-- Global site tag (gtag.js) - Google Analytics -->
+<script async src="https://www.googletagmanager.com/gtag/js?id=G-2T4F5GFLP5"></script>
+<script>
+    var localAddrs = ["localhost", "127.0.0.1", ""];
+
+    // make sure we don't activate google analytics if the developer is
+    // inspecting the book locally...
+    if (localAddrs.indexOf(document.location.hostname) === -1) {
+        window.dataLayer = window.dataLayer || [];
+        function gtag() { dataLayer.push(arguments); }
+        gtag('js', new Date());
+
+        gtag('config', 'G-2T4F5GFLP5');
+    }
+</script>
+
 </html>