If you'd like to specify different tempos and time signature within a song - so say 16 bars of 4/4 at 118 bpm, followed by 8 bars of 3/4 at 125bpm.
You can do this on iOS now!
1. Switch to the Playlist interface, enable "Auto Switch" Mode button.
2. Expand the Playlist interface into the full-screen mode. You'll find that you're able to set bars for each song.
After that, you'll find the metronome will continue to play the next song in the playlist.