Looking at this:
https://docs.godotengine.org/en/stable/classes/class_engine.html#class-engine-method-get-idle-frames

Does this give me the number of frames processed since the game started? Or does it only count frames in which some qualification for "idle" is met?

I'm trying to set up a thing where I need the total number of frames passed since the game started, and if possible I wish to use built-in stuff rather than making a counter on my own.

cybereality what would be the difference?
Also, the manual says that in some situation get_frames_drawn() would always return 0, is that situation something that can occur during typical usage of the game?

That is for servers. On a normal game it adds one every frame that is rendered.