Filter custom post type by tag

filter_cpt_by_tag.php

<?php
/**
 * Adds admin area filtering for custom post type tags.
 *
 * This is separate because, for some reason, the taxonomy
 * is 'post_tag' but the query var is 'tag'.
 *
 * @param string $post_type The post type slug.
 * @return void
 */
function filter_cpt_by_tag( $post_type ) {

	// Apply this only on a specific post type
	if ( 'example_post_type' !== $post_type ) {
		return;
	}

	$params = array(
		'show_option_all' => get_taxonomy('post_tag')->labels->all_items,
		'hide_empty' => true,
		'hierarchical' => 1,
		'show_count' => true,
		'orderby' => 'name',
		'name' => 'tag',
		'value_field' => 'slug',
		'taxonomy' => 'post_tag',
	);

	if ( isset($_GET['tag']) ) {
		$params['selected'] = $_GET['tag'];
	}

	wp_dropdown_categories($params);

}
add_action( 'restrict_manage_posts', 'filter_cpt_by_tag' );