Verify cache walker removes expected empty directories

Relates to #2292 (comment 271477554)

The cache walker has been updated to remove empty directories as it crawls the filesystem. This should be verified in production with the assistance of an SRE.

The following steps should be executed:

1. Analyze cache walker activity to determine roughly when it will execute again
2. Have an SRE list the empty directories on a Gitaly storage right before the next cache walker run:
   • bundle exec knife ssh role:gprd-base-stor-gitaly "sudo find /var/opt/gitlab/git-data/repositories/+gitaly/state -empty -type d | wc -l"
3. Compare the prometheus counters with the expected values:

Blocked until this change is deployed to master: !1769 (merged)