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`register_meta()` has been altered to accept an array of arguments as the third parameter in order to support its usage beyond XML-RPC, notably in the REST API and other projects that may build on top of meta, such as a potential Fields API. Arguments are whitelisted to reserve the right for core to add more later. New functions added to complement this expansion are: * `registered_meta_key_exists()` * `unregister_meta_key()` * `get_registered_meta_keys()` * `get_registered_metadata()` * A "private" function for the aforementioned whitelisting. There still need to be lots of tests written for previous and new behaviors, and many things are subject to change. Maybe things will explode. #yolo props jeremyfelt, ericlewis, sc0ttkclark, helen, rmccue, ocean90, voldemortensen. see #35658. git-svn-id: https://develop.svn.wordpress.org/trunk@37924 602fd350-edb4-49c9-b593-d223f7449a82