Buffer memory usage for custom allocations is reported as 0

[jhorstmann]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

Describe the solution you'd like

I'd like to keep track of the buffer length in Buffer::from_custom_allocation in order to more accurately report the memory usage of arrays created from custom allocations.

Describe alternatives you've considered

Additional context

[tustvold]

label_issue.py automatically added labels {'arrow'} from #5347