[Feature flag] Enable Ruby reference-transaction hook

What

This issue tracks progress in enabling the Ruby reference-transaction hook, which would cause us to use the reference-transaction hook from Ruby RPCs which make use of with_hooks.

Owners

• Team: Gitaly
• Most appropriate slack channel to reach out to: #g_gitaly
• Best individual to reach out to: @pks-t

Expectations

What release does this feature occur in first?

%13.4

### What are we expecting to happen?

Ruby RPCs which go via Praefect will start calling out to the reference-transaction hook. Actual transactions would only start to be used for the OperationService for the following RPCs:

• UserCreateBranch
• UserCreateTag
• UserDeleteBranch
• UserDeleteTag
• UserUpdateBranch

What might happen if this goes wrong?

Increased error ratios for above RPCs. Failing transactions.

What can we monitor to detect problems with this?

• feature-specific dashboards (https://dashboards.gitlab.net/d/000000199/gitaly-feature-status?orgId=1&from=1603363523990&to=1603371615419&var-environment=gprd&var-method=UserMergeToRef&var-prometheus=prometheus-01-inf-gprd)
• transactions in the Praefect dashboard https://dashboards.gitlab.net/d/8EAXC-AWz/praefect?orgId=1&refresh=30s

Roll Out Steps

• Read the documentation of feature flags
• Enable on staging
• Test on staging
• Ensure that documentation has been updated
• Enable on GitLab.com for individual groups/projects listed above and verify behaviour
• Announce on the issue an estimated time this will be enabled on GitLab.com
• Enable on GitLab.com by running chatops command in #production
• Cross post chatops slack command to #support_gitlab-com and in your team channel
• Announce on the issue that the flag has been enabled
• Remove feature flag and add changelog entry