I just tested this on a fresh user account.
I haven’t got a Mac, but on Windows’s default Renoise installation this function is associated to the "" key, at left of “1” key (not numpad)
I noticed that too, having a mac myself. I remember it’s an ASCII char, using ZZT for a while planted the ASCII char set firmly in my mind. I don’t think much about it though; I don’t use the metronome really. Try looking at the key prefs and look for the metronome. I’ll look too.
On my keyboard, ` is on the left of 1 (~ if shifted).