[fenix] For https://github.com/mozilla-mobile/fenix/issues/3351 - Adds docs for the sync_auth telemetry

docs/metrics.md

@@ -594,4 +594,60 @@ tracking_protection</td>
 </table>
 </pre>
 
+## sync_auth
+
+<pre>
+<table style="width: 100%">
+    <tr>
+        <th>key</th>
+        <th>type</th>
+        <th>description</th>
+        <th>data deview</th>
+        <th>extras</th>
+        <th>expires</th>
+    </tr>
+    <tr>
+        <td>opened</td>
+        <td>event</td>
+        <td>A user opened the sync authentication page</td>
+        <td><a href="https://github.com/mozilla-mobile/fenix/pull/2745#issuecomment-494918532">link</a></td>
+        <td></td>
+        <td>2020-03-01</td>
+    </tr>
+    <tr>
+        <td>closed</td>
+        <td>event</td>
+        <td>A user closed the sync page</td>
+        <td><a href="https://github.com/mozilla-mobile/fenix/pull/2745#issuecomment-494918532">link</a></td>
+        <td></td>
+        <td>2020-03-01</td>
+    </tr>
+    <tr>
+        <td>sign_in</td>
+        <td>event</td>
+        <td>A user pressed the sign in button on the sync authentication page</td>
+        <td><a href="https://github.com/mozilla-mobile/fenix/pull/2745#issuecomment-494918532">link</a></td>
+        <td></td>
+        <td>2020-03-01</td>
+    </tr>
+    <tr>
+        <td>scan_pairing</td>
+        <td>event</td>
+        <td>A user pressed the scan pairing button on the sync authentication page</td>
+        <td><a href="https://github.com/mozilla-mobile/fenix/pull/2745#issuecomment-494918532">link</a></td>
+        <td></td>
+        <td>2020-03-01</td>
+    </tr>
+    <tr>
+        <td>create_account</td>
+        <td>event</td>
+        <td>A user pressed the create account button on the sync authentication page</td>
+        <td><a href="https://github.com/mozilla-mobile/fenix/pull/2745#issuecomment-494918532">link</a></td>
+        <td></td>
+        <td>2020-03-01</td>
+    </tr>
+</table>
+</pre>
+
+
 ## Metrics