This is the most basic implementation (I don’t have a noisy scene to hand) but this should give you a quick idea of what you’re looking at.
The crucial bits are as follows:
Take an output from your source footage to an RGB to LAB node. LAB gives you an output with a lightness channel and two “colour” axes - A and B. We only need the Lightness (L) channel so I used a shuffle note and direct the R, G and B so you have A.r, B.r and G.r to make sure the output reflects the lightness.
This becomes your mask which you feed to the Denoiser - note that you might need to tweak the output from the shuffle node to get the right effect. You can alter the mask by simply inverting, using threshold or even colour correction to alter the strength of effect according to your needs.
Don’t forget you can feed the output from the shuffle node to a viewer node to make sure you’re getting a nice greyscale image.
@omar is more experienced than I am so he might have a more detailed detailed version but this should get you started I hope.