[Feature flag] Enable Go implementation of UserUpdateBranch

What

Enable the :gitaly_go_user_update_branch feature flag ...

Owners

• Team: Gitaly
• Most appropriate slack channel to reach out to: #g_create_gitaly
• Best individual to reach out to: @avar

Expectations

Changes the underlying codepath of UserUpdateBranch from Ruby to Go.

What release does this feature occur in first?

v13.9.0

What are we expecting to happen?

Things should Just Work.

What might happen if this goes wrong?

The remote mirroring of repositories might have issues, AFAICT that's the only user of the API call. If it does the current behavior is just to retry later, so I'm not too worried about this.

What can we monitor to detect problems with this?

• staging dashboard: https://dashboards.gitlab.net/d/000000199/gitaly-feature-status?orgId=1&refresh=30s&var-environment=gstg&var-method=UserUpdateBranch&var-prometheus=prometheus-01-inf-gstg
• prod dashboard: https://dashboards.gitlab.net/d/000000199/gitaly-feature-status?orgId=1&refresh=30s&var-environment=gprd&var-method=UserUpdateBranch&var-prometheus=prometheus-01-inf-gprd

Beta groups/projects

If applicable, any groups/projects that are happy to have this feature turned on early. Some organizations may wish to test big changes they are interested in with a small subset of users ahead of time for example.

• gitlab-org/gitlab / gitlab-org/gitaly projects
• gitlab-org/gitlab-com groups
• ...

Roll Out Steps

• Read the documentation of feature flags
• Add featureflagstaging to this issue (howto)
• Is the required code deployed? (howto)
• Do we need to create a change management issue? (howto)
• Enable on staging (howto)
• Test on staging (howto)
• Announce on this issue an estimated time this will be enabled on GitLab.com
• Add featureflagproduction to this issue
• Enable on GitLab.com by running chatops command in #production (howto)
• Did you set the feature to both 100% and true (howto)
• Cross post chatops slack command to #support_gitlab-com and in your team channel
• Announce on the issue that the flag has been enabled
• Submit a MR to have the feature OnByDefault: true and add changelog entry (howto)
• Have that MR merged
• Possibly wait for at least one deployment cycle (howto)
• Submit a MR to remove the pre-feature code from the codebase and add changelog entry (howto)
• Have that MR merged
• Remove the feature flag via chatops (howto)
• Close this issue