Make $key argument optional in get_*_meta()

[scribu]

Altough the $key argument is optional in get_metadata(), it's not optional in the wrapper functions for some reason.

This should be fixed.

Related: #15483

Take care with renaming @param string $meta_key while not renaming $key in the functions' signatures. Looks like a copy&paste glitch.

$meta_key says more btw, so I wouldn't mind if you even rename the parameter names in the signature.

Rename $key to $meta_key

No objection here.

Those who can't search through trac tickets are bound to re-create them: #19882