How do I tell the iOS Music app to regenerate its metadata? To clear storage space in order to upgrade to iOS 8, I went to Settings/General/Usage/Manage Storage/Music on my iPad and deleted all my music (I have it all stored in iTunes on my Mac at home.)

I got the space back and installed iOS 8 over-the-air. But now when I go to the Music app, it looks like my music library is still there—the metadata (including album art) still shows. When I tap on a song, the name greys out and a "Stop" icon in a circle appears on the right hand side of the table.

(When I go into Settings/General/Usage/Manage Storage/Music, it tells me all my songs are still taking up 41.2GB, and lists each artist with their individual usages.)

How can I force the iPad to dump and regenerate its cache of Music.app metadata?
If you have iTunes Match, you would simply disable that on the device, power it down and then sign back in to iTunes Match in the settings app under Music.

If you don't have iTunes Match, connect the iPad to iTunes and then sync the music back over USB or Wifi if you enable wireless syncing of that device.