Add `used` memory size to `FairSpillPool`

[erenavsarogullari]

Is your feature request related to a problem or challenge?

Currently, both GreedyMemoryPool and UnboundedMemoryPool track used memory size in addition to pool_size. Adding used memory size to FairSpillPool can also be useful. This is follow-up effort for #20387 (comment).

Describe the solution you'd like

To add used memory size to FairSpillPool.

Describe alternatives you've considered

No response

Additional context

No response

[erenavsarogullari]

take