Expose METALLIC in the light() function for spatial shaders

[pezcode]

Describe the project you are working on:
Writing a spatial shader with custom lighting code

Describe the problem or limitation you are having in your project:
light() doesn't expose a METALLIC built-in which is needed for correct lighting calculation.

Describe the feature / enhancement and how it helps to overcome the problem or limitation:
My workaround is using metallic which is a variable in Godot's shader code. This could break with any other version of Godot, having a proper built-in would be preferrable.

Describe how your proposal will work, with code, pseudocode, mockups, and/or diagrams:
This would just be another built-in defined exactly like ROUGHNESS.

If this enhancement will not be used often, can it be worked around with a few lines of script?:
Not really, no.

Is there a reason why this should be core and not an add-on in the asset library?:
It seems like a nice minor QoL improvement, and I don't know enough about add-ons to say if replacing the core shaders is possible or feasible.

Thanks!

PS: I'm not exactly familiar with Godot internals, but this seems simple enough that I could send a PR myself, if this is deemed useful.

[clayjohn]

I agree, METALLIC should be exposed as a read-only variable to light shaders.

Youll have to add it here https://github.com/godotengine/godot/blob/b5eea37fb04755e71c541727c1edf3a51e8fe30f/servers/rendering/shader_types.cpp#L150

And in the docs here https://docs.godotengine.org/en/latest/tutorials/shading/shading_reference/spatial_shader.html