housekeeping: Fix uncollected time-since-last-optimization metric

We've recently introduced a new metric that tracks the time since the last optimization for a particular data structure has been performed in d14f2ef9 (housekeeping: Add metric to track time since last full repack, 2023-03-17). Unfortunately, we forgot to actually wire up the metric, making it effectively useless.

Fix this bug by correctly collecting the metric.

Changelog: fixed