Instructions for using the custom setting for rpiCameraMetering?

[mattlazarowitz]

Question

I'd like to try setting up a custom meting region for the camera on my Pi. I'm using a camera with a switchable IR cut filter along with IR illumination that runs in low light conditions. I'm trying to see if I can knock out a region in the scene that is pretty reflective in IR to see if that improves the low light performance for the parts of the scene I'm actually interested in.
Switching between presets has helped some but I think I can get the best results by metering based on the most important region in the scene.

Does a custom region have to be set up in the tuning file, or can I somehow supply the matrix in mediamtx.yml?