I don't remember which version introduced this. But the current version of Sonar allows the metronome to be subdivided by 1/2, 1/3, or 1/4. Just click on the metronome icon in the control bar.
It was not always this way. However I never use the metronome anymore. I find simple Step Sequencer patterns to be a much more useful alternative (especially for triple meters like 6/8 or syncopated beats).