Add Login/Username/Logout(as submenu) to main navigation

functions.php

function add_login_logout_to_menu( $items, $args ) {
	if ( $args->theme_location != 'primary' ) {
		return $items;
	}

	if ( !is_user_logged_in() ) {
		$items .= '<li><a href="'.wp_login_url().'" title="Login">Login</a></li>';
	} else {
		$current_user = wp_get_current_user();
		$items .= '<li class="menu-item-has-children"><a href="#">'. $current_user->display_name .'</a><ul class="sub-menu">';
		$items .= '<li><a href="'.wp_logout_url( home_url() ).'">Logout</a></li>';
		$items .= '</ul></li>';
	}

	return $items;
}

add_filter( 'wp_nav_menu_items', 'add_login_logout_to_menu', 199, 2 );