Add unreserved_space column to system.disks table

Why it matters

This feature allows users to monitor and understand the free disk space available for use beyond the reserved space, enabling better disk space management and planning.

How to use it

Query the system.disks table and include the unreserved_space column to see how much space on each disk is not taken by reservations, for example:

SELECT name, unreserved_space FROM system.disks;