fix old typo
push existing allocation_path down to _get_allocation_without_lock

Checking that an allocation_path does not exist on every candidate
volume is very slow due to having to mount the volume and stat a path.
This pushes the check down to where each candidate volume is attempted
rather than checking all candidate volumes.
add metric: disk.allocation.child_allocation_query
add metric: disk.allocation.get_parent_allocation
add metric: disk.allocation.require
extract method: _existing_allocation_path_check
allow expected/got count to be shown in diag