I'm super sorry if this offends anyone, but I just can't help but hear "metaballs" as a fake Italian Mario accentt like, "What-ah happen to my meat-ah balls?"
I tried to create metaballs in Godot about 2 years ago, but if I remember correctly, there is no inbuilt way to achieve this easily. CSGMesh is not useful anyways.
Back then I tried around with different screen space shaders but ultimately chose a different look for my project and thus abandoned this pursuit.
So I am pretty sure it is doable, but it is not going to be straightforward.