Can I add User-ID to the Analytics code?

Hi, I see it’s easy to add the User-ID as specified by Google, I adapted this code.

and added it to my child theme via functions.php, like this:

//* Add User-ID for Analytics for logged in users
add_action( 'wp_head', 'ux_userID', 99 );
function ux_userID() {
	if (is_user_logged_in()) {
        $user = wp_get_current_user();
        $userName = $user->user_login;
    }
	if (isset($userName)) :
  ?>
  <!-- User-ID for Analytics -->
   <script type="text/javascript">
        ga('set', 'userId', <?php echo(json_encode($userName)); ?>); // Set the user ID using signed-in user_id.
   </script>
  <!-- End User-ID for Analytics -->
  <?php
	endif;
}

However, it is not much use for the MonsterInsights since it’s is separate from the generated Analytics code.

I see you can add code to MonsterInsights via Settings > Tracking > Compatability however you can’t add php code there.

So how can this be done? It looks like either pay $99 USD per annum or add Analytics manually without a plugin?