There are a couple of ways... this is the way I'd go about doing it:
1. Create a new audio track.
2. Open the Mixer and choose to see "All"
3. Inst 256 will be the klopfgeist ('knocking ghost' in German) which is the Metronome. Create a send to an empty Bus (e.g. Bus 10) and set the level as desired.
4. On the audio channel strip, set the input to the Bus above.
5. Turn on the Metronome & Record enable the Audio Channel and press record on the Transport.
You have now recorded the metronome to an audio file.
Remember to reset the send from Inst 256 afterwards :)