update_metadata() passes only the first meta_id

[scribu]

Code to reproduce:

function update_metadata_action_test($meta_id) {
	var_dump($meta_id);
}
add_action('update_post_meta', 'update_metadata_action_test');

add_metadata('post', 1, 'mykey', 'value1');
add_metadata('post', 1, 'mykey', 'value2');

update_metadata('post', 1, 'mykey', 'new value');

Expected result:

Array ( [0] => 101 [1] => 102 )

Actual result:

string(3) "101"

Two solutions:

1. update_usermeta() only updates the first value that it finds

2. update_usermeta() uses $wpdb->get_col() instead of $wpdb->get_var()

The first would create more severe backward compatibility issues than the second.

Related: #10905

Come to think of it, I have no ideea why $meta_id is passed at all (and as the first parameter at that). It's not accesible from anywhere else in the API.

Needs to be handled with #10905 I'd think.