add more properties to add-meta-tags properties

gistfile1.php

<?php
function pg_filter_metatags( $tags ) {
	$tags = str_replace( 'meta name="description"', 'meta name="description" itemprop="description"', $tags );
	$tags = str_replace( 'meta name="keywords"',    'meta name="keywords" itemprop="keywords"',       $tags );
	return $tags;
}
add_filter( 'amt_metatags', 'pg_filter_metatags' );