How to include metronome in exported file

Not possible directly.

The way I usually deal with that type of situation is to create a separate click track using a simple stick/block hit loop.

This has come up many times - I don't believe there's a way to route the internal audio metronome to your output WAV or MP3 rendering.  I think the only way you can accomplish this is by adding MIDI track, routing your metronome to that MIDI track and route the output of the MIDI track to a soft synth (or to an external hardware synth) to produce the click sound audio, which you can then record / route as any other audio signal into your mix.

It would seem to me that the upcoming "Virtual Patch Points" feature would be a way to loop the audio from the metronome back to a track and capture it that way...which would be great...until you decide to change the tempo.  Then you'd have to recapture it.
 
Maybe the stunt where you create a chopped up groove clip and drag it out is the best solution (as referenced in the link above).