Further improved availability recovery (#3711)

* WiP.

* Things compile.

* cargo fmt

* Passing tests + fix warnings.

* Metrics for availability recovery.

* Basic test.

* Fix typos and actually check for overflow.

* cargo fmt

* Register metrics.

* More tests.

* Fix warning.

* cargo +nightly fmt

* Fix metrics

* Get rid of unsafe.

* tabify

* spellcheck

Co-authored-by: Andronik Ordian <write@reusable.software>
Co-authored-by: Bastian Köcher <info@kchr.de>