adjusting stem length in metronome mark?

[Josiah Boothby]

I used to know how to do this, I think, but...how do I adjust the stem length in a metronome mark? Seems like I should be able to do something like

\override MetronomeMark.stem-length = xyz

but that *specifically* doesn't appear to be available.

Something involving .add-stem-length perhaps?

—J