How can I add meta tags in the WordPress header?

Since you are you using Yoast WordPress SEO plugin you can utilize its hook action wpseo_opengraph to add your custom meta tag property.

Add the following code to your theme functions.php file:

function wpse213_article_tag(){
    global $post;
    
    if($post->post_type == 'post') {    
        if($tags = get_the_tags($post->ID)) {
            foreach($tags as $tag) {
                echo '<meta property="article:tag" content="' . esc_attr($tag->name) . '" />' . "\n";
            }
        }
    }
}
add_action( 'wpseo_opengraph', 'wpse213_article_tag', 10 );