WordPress at a glance
filter is not described

postmeta_form_keys filter-hook . WP 4.4.0

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.

Usage

add_filter( 'postmeta_form_keys', 'filter_function_name_4551', 10, 2 );
function filter_function_name_4551( $keys, $post ){
	// filter...

	return $keys;
}
$keys(array/null)
Pre-defined meta keys to be used in place of a postmeta query.
Default: null
$post(WP_Post)
The current post object.

Where the hook is called

meta_form()
postmeta_form_keys
wp-admin/includes/template.php 612
$keys = apply_filters( 'postmeta_form_keys', null, $post );

Where the hook is used (in WP core)

Does not used.