Prints the form in the Custom Fields meta box.

function meta_form( $post = null ) {
	global $wpdb;
	$post = get_post( $post );

	/**
	 * Filters values for the meta key dropdown in the Custom Fields meta box.
	 *
	 * Returning a non-null value will effectively short-circuit and avoid a
	 * potentially expensive query against postmeta.
	 *
	 * @since 4.4.0
	 *
	 * @param array|null $keys Pre-defined meta keys to be used in place of a postmeta query. Default null.
	 * @param WP_Post    $post The current post object.
	 */
	$keys = apply_filters( 'postmeta_form_keys', null, $post );

	if ( null === $keys ) {
		/**
		 * Filters the number of custom fields to retrieve for the drop-down
		 * in the Custom Fields meta box.
		 *
		 * @since 2.1.0
		 *
		 * @param int $limit Number of custom fields to retrieve. Default 30.
		 */
		$limit = apply_filters( 'postmeta_form_limit', 30 );

		$keys = $wpdb->get_col(
			$wpdb->prepare(
				"SELECT DISTINCT meta_key
				FROM $wpdb->postmeta
				WHERE meta_key NOT BETWEEN '_' AND '_z'
				HAVING meta_key NOT LIKE %s
				ORDER BY meta_key
				LIMIT %d",
				$wpdb->esc_like( '_' ) . '%',
				$limit
			)
		);
	}

	if ( $keys ) {
		natcasesort( $keys );
		$meta_key_input_id = 'metakeyselect';
	} else {
		$meta_key_input_id = 'metakeyinput';
	}
	?>