Thanks for your replies.
I would have preferred
Karyn's suggestion to send it to a different bus and export it, but this doesn't work. You get an empty file, apparently because the metronome is turned off for exporting even if active during playback. It's the same situation when you export using audible bounce/"what you hear" ... the metronome is turned and not sounding during export. I reckon this is a reasonable safety measure (and see no need for this to be changed) ...
The solution that finally worked (and now is simple/fast to use when needed) was to
create a groove clip containing the metronome for a long measure i.e. dragged and dropped the higher click for beat 1 and several lower clicks for the following beats, bounced to single clip, sliced it up in loop editor and saved it as audio groove clip. Now when I need it I drag it into an audio track, truncate it according to measures used (4/4 or 6/4 or whatever), discard the chopped rest and use groove clip looping to extend across entire song ... works nice at tempo changes and at export ...