Remove unnecessary "type" attribute from MI script code to keep HTML validators happy. Second file adds async attribute to external gtag script.

remove_type_attr_1148073.php

<?php // Ignore this line if copying to child theme's functions.php

/** Remove the unnecessary "type" attribute from MI script code by leaving it out of the filter. */
function remove_type_attr_1148073() {
	return array(
		'data-cfasync' => 'false' // Ask CloudFlare Rocket Loader not to execute.
		);
}
add_filter( 'monsterinsights_tracking_analytics_script_attributes', 'remove_type_attr_1148073' );

/** Add to child theme functions.php */

remove_type_attr_add_async.php

<?php // Ignore this line if copying to child theme's functions.php

/** 
 * Remove the unnecessary "type" attribute from MI script CODE and add the "async" attribute
 * to gtag.js script code. 
 */
function remove_type_attr_add_async() {
	return array(
		'data-cfasync' => 'false', // Ask CloudFlare Rocket Loader not to execute.
		'async', // Adds the async attribute to the gtag library script tag.
		// 'defer' SHOULD AVOID unless pagespeed is more important than accurate stats. If used, must remove async.
		);
}
add_filter( 'monsterinsights_tracking_analytics_script_attributes', 'remove_type_attr_add_async' );

/** Add to child theme functions.php */

Async added.

Note that async added to inline scripts has no effect.