title: Configure Firebase Analaytics + Google Tag Manager (GTM) per build variant
body: <p>Before Firebase Analytics became available we use a multi flavour, multi build type Android Gradle project setup, and supply a different GTM container ID for each build variant, as follows:</p>
  
  <pre><code>TagManager.getInstance(context)
          .loadContainerPreferNonDefault(BuildConfig.GTM_CONTAINER_ID, -1);
  TagManager.getInstance(context).getDataLayer().pushEvent(eventName, eventData);
  </code></pre>
  
  <p>where Gradle would inject different <code>GTM_CONTAINER_ID</code> per build variant.</p>
  
  <p>How do we achieve the same with Firebase Analytics + GTM? According to <a href="https://developers.google.com/tag-manager/android/v5/" rel="noreferrer">docs</a>, we need to download:</p>
  
  <ul>
  <li>a GTM container file from GTM dashboard <a href="https://developers.google.com/tag-manager/android/v5/#add-gtm-to-project" rel="noreferrer">[1]</a></li>
  <li>a google-services.json file from Firebase console <a href="https://firebase.google.com/docs/analytics/android/start/" rel="noreferrer">[2]</a></li>
  </ul>
  
  <p>and then just start firing events with this:</p>
  
  <pre><code>FirebaseAnalytics.getInstance(context).logEvent(eventName, bundle);
  </code></pre>
  
  <p>Where do we specify the GTM container ID to be used? Or is it auto derived by the file name we download from GTM dashboard and put under <code>assets/containers</code>? If so, how do we use different GTM configuration per build variant as we do with GTM legacy Android container?</p>