WordPress at a glance
action is not described

add_(meta_type)_meta action-hook . WP 3.1.0

Fires immediately before meta of a specific type is added.

The dynamic portion of the hook, $meta_type, refers to the meta object type (comment, post, term, or user).

Usage

add_action( 'add_(meta_type)_meta', 'action_function_name_6574', 10, 3 );
function action_function_name_6574( $object_id, $meta_key, $meta_value ){
	// action...
}
$object_id(int)
Object ID.
$meta_key(string)
Meta key.
$meta_value(mixed)
Meta value.

Where the hook is called

add_metadata()
add_(meta_type)_meta
wp-includes/meta.php 96
do_action( "add_{$meta_type}_meta", $object_id, $meta_key, $_meta_value );

Where the hook is used (in WP core)

Does not used.