Gitaly shard rebalancing

Problem to solve

Over time Gitaly shards will become out of balance, particularly when new shards are added. To fix this, I need to use the API to rebalance the shards.

Further details

GitLab have a custom script in https://gitlab.com/gitlab-com/gl-infra/production/issues/875 that rebalances shards to reduce disk usage. We should provide an API or tool in the UI so that customers can do this more easily.

Proposal

Add the ability to rebalance Gitaly shards based on disk usage to the admin interface

Links / references