The new wallpaper picker will add several new functionalities that need metrics (the list may be expanded later):

1) 'My photos' section: see how many local files a user have under the
   Downloads folder, so that we can decide if we want to fetch the
   file names incrementally.

2) 'Preview wallpaper': see how many users prefer to preview the
    wallpaper before actually setting it.

3) 'Learn more' links: see how many users are actually interested in
    knowing more about the photographer/info of the wallpaper.