chore(storage-monitor): improve `free_space` calculation and cli default value (#14133)

* chore(storage-monitor): fix free_space calculation

* add `Result` type

* add docs

* update `polling_period` default value to `6`

* Update client/storage-monitor/src/lib.rs

Co-authored-by: Michal Kucharczyk <1728078+michalkucharczyk@users.noreply.github.com>

* Apply suggestions from code review

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Co-authored-by: Michal Kucharczyk <1728078+michalkucharczyk@users.noreply.github.com>
Co-authored-by: Bastian Köcher <git@kchr.de>