If you're talking about having instrumental tracks and a metronome coming from a single audio interface, you can do it but need enough outputs from that interface. If you want stereo tracks, you should need 3 outputs because a metronome output should only require mono (stereo inst (2) + metronome (1) = 3). If you also want to use a guide track or such and keep those separate from everything else, it'll need its own output—4 outputs.
If you're talking about having a metronome from an isolated source (no extra stuff like stereo tracks running from the interface), you just treat it like anything else and keep it out of the mains but leaving it in the monitors.
