Problem
The metronome does not appear in the list of busses available for export.
If the metronome is routed to a bus that can be exported, the metronome does not play during export so still does not get exported.
 
I can see this is a safety measure to prevent tech support requests complaining that the metronome is bleeding all over the exported tracks, but there are many uses an exported metronome could be put to, including creating click/cue tracks for live use.
 
There are alternatives to using the metronome, including creating a dedicated midi track that re-creates the metronome, but that is duplicating something which is already built into Sonar and the metronome automatically follows things like time signature changes whereas a midi track would need to be manually edited to allow for them.
 
Solution 1.
A large button (default OFF, as per current operation) on the metronome console strip, track lane, track inspector, marked "Export Enable". Have it glow very bright red when enabled as a warning.  Enabling export allows the metronome to sound during export so it can be routed as any normal audio channel.
 
Solution 2.
Include the metronome bus in the list of available sources for export.  Allow the metronome to sound during export if the metronome bus is selected as a source.  If the metronome channel has been routed to anywhere other than a direct soundcard output (eg: Master Bus) display a warning (with a cancel option) that exported buses may include the metronome mixed in.
 
Solution 3.
Have the metronome be a prompt on export to offload with the current stem functionality (I.e. force it into its own stem if chosen, regardless of project routing).  (Idea provided by Mettelus)

Or have the metronome be a prompt on export to offload with the current stem functionality (I.e. force it into its own stem if chosen, regardless of project routing).

I definitely vote for this as I was "struggling" with this just yesterday.
 
For all who can't wait and need that functionality - here's a workaround that I used involving groove clips: http://forum.cakewalk.com/FindPost/3228675
 
As regards suggested ways of implementation - I think solution #2 would be OK because default projects already route the metronome to a separate bus (and I consider this "best practise"), so allowing export of the metronome bus would be sufficient.
 
A warning when it might bleed into the master bus during a fast bounce export would be nice, but please allow to switch that warning off in the INI files (e.g. like the silent bus warning) because usually when it comes to exporting, tracking/comping/fixing is long done and the metronome no longer enabled during export ...

Being able to bounce the metronome would be pretty handy.  So would being able to load your own metronome sounds
 
 

BobF
So would being able to load your own metronome sounds

 
Yes, a slight improvement of metronome would be rather easy and useful.
 
Five sounds
accent, 1/4, 1/8,  1/8 triplet, 1/16 - and volume for each.
 
So simple, so effective.
My reference is Korg Beatlab metronome.
 
Then you have support to run 4/4 with triplets - not going over 12/8 time sig and change tempo so you get the tempo you want. Now you want 100 bpm 4/4 triplet - you need to set 12/8 and tempo 150.
 
Now I record click track from drum engine, which have the above - and align to grid.