begin metadata migration. still need to adapt old client code
begin metadata migration. still need to adapt old client code

simon-weber/gmusicapi,TheOpenDevProject/gmusicapi,jimyx17/gmusic,nvbn/Unofficial-Google-Music-API,simon-weber/Unofficial-Google-Music-API,dvirtz/gmusicapi,tanhaiwang/gmusicapi,dvirtz/gmusicapi,peetahzee/Unofficial-Google-Music-API,thebigmunch/gmusicapi