Document How to Ensure that Online Garbage Collection is Healthy for Self Managed Users
Context
In gitlab#423459 (comment 1712901515) we realized that we do not have documentation for self-managed uses on how to monitor online GC.
Solution
We should create a section in the container registry administration docs to this effect, and create a post import section in the import guides which points to this documentation and gives advice on what to look for after a completed import. In particular, we should point out that the database will experience a period of high load as the queues drain.