Send user agent as a custom dimension via MonsterInsights action hook

send-user-agent-custom-dimension.php

<?php // Remove this line when adding to child functions.php

/**
 * This example sets custom dimension #14 to be the HTTP
 * user agent.
 *
 * This is compatible with MonsterInsights gtag code. This filter supercedes 
 * the monsterinsights_frontend_tracking_options_analytics_before_pageview
 * filter.
 *
 * Change the number 14 to your custom dimension index number in GA.
 */
add_filter(
  monsterinsights_frontend_tracking_options_persistent_gtag_before_pageview,
  function( $options ) {
	  $options[ 'dimension14'] = $_SERVER['HTTP_USER_AGENT'];
	  return $options;
  }, 
  20 // priority
);

GA doc for creating a custom dimension in a UA property: https://support.google.com/analytics/answer/2709829