[Bullet] Performance monitor for 3D physics not working for Bullet

[nadnam]

Godot version:
v3.0-stable_x11.64

Issue description:
Performance monitor for 3D physics not working for Bullet, all values are 0. This issue was mentioned earlier here, among other issues: #15975

Steps to reproduce:
Run attached project. Monitor values are shown in window title. Cube will fall on surface and sleeping is disabled, so there should be 1 active object/collision pair/island. Run with GodotPhysics to see correct values.

Minimal reproduction project:
BUgTest.zip

[reduz]

Well, if Bullet does not expose these, there is not much that can be done, but is this possible, @AndreaCatania ?

[AndreaCatania]

Yes bullet expose these stats, I need to implement it, but I'm moving it for 3.2 :)

[KoBeWi]

Still valid in 5ab727b

[Klowner]

I figured I'd give this a shot. From what I can see, it doesn't look like Bullet exposes these stats directly (unless I simply can't find it in the Bullet source), but it looks simple enough to count the number of active non-static bodies (adds an O(n) loop over the bodies) for active object count. For collision pairs there's a counter for the total number of collision manifolds. But the island count doesn't appear to be exposed unless you count the number of unique island tags used in the bodies after a stimulation step.

I'd be happy to implement if any of that sounds sensible, any input is greatly appreciated!