Be able to track request queuing for IDAs.

playbooks/roles/edx_django_service/templates/edx/app/nginx/sites-available/app.j2

@@ -84,6 +84,10 @@ server {
     proxy_set_header X-Forwarded-Port $http_x_forwarded_port;
     proxy_set_header X-Forwarded-For $http_x_forwarded_for;
     {% endif %}
+
+    # newrelic-specific header records the time when nginx handles a request.
+    proxy_set_header X-Queue-Start "t=${msec}";
+
     proxy_set_header Host $http_host;
 
     proxy_redirect off;