Fibre objectState, finalizer and callbacks metric

[swapdonkey]

Hi,
Our application had a memory leak similar to typelevel/fs2#3404
To make memory leaks like this more obvious as well as gain some additional visibility into cats effect would it be possible to add fiber level count metrics for objectState, finalizer and callbacks in IOFiber:

cats-effect/core/shared/src/main/scala/cats/effect/IOFiber.scala

Lines 87 to 89 in fd2f83f

	private[this] val objectState: ArrayStack[AnyRef] = ArrayStack()
	private[this] val finalizers: ArrayStack[IO[Unit]] = ArrayStack()
	private[this] val callbacks: CallbackStack[OutcomeIO[A]] = CallbackStack.of(cb)