When the monitor is started, the first disk utilization value is
obtained from the actual host filesystem and not from the fake
space source function.
Thus, register a fake space source function before the monitor
is started.
Fixes: https://github.com/scylladb/scylladb/issues/26036
Backport is not required. The test has been added recently.
Closesscylladb/scylladb#26054
Introduce the `subscribe` method to disk_space_monitor, allowing clients to
register callbacks triggered when disk utilization crosses a configurable
threshold.
The API supports flexible trigger options, including notifications on threshold
crossing and direction (above/below). This enables more granular and efficient
disk space monitoring for consumers.