add_filter( 'woocommerce_product_filters', 'filter_by_custom_attributes_dashboard_products' );
function filter_by_custom_attributes_dashboard_products( $output ) {
global $wp_query;
$output .= wc_product_dropdown_categories( array(
'show_option_none' => 'Filter by product attributes',
'product_attributes' => 'attributes',
'name' => 'brand-name',
'selected' => isset( $wp_query->query_vars['attributes'] ) ? $wp_query->query_vars['attributes'] : '',
) );
return $output;
}