FeatureFlag: roll out of 'gitaly_go_fetch_remote' - port of FetchRemote RPC from ruby to go

What

Enabling of :gitaly_go_fetch_remote feature flag.
Corresponding MR: !2724 (merged)

Owners

• Team: Gitaly
• Most appropriate slack channel to reach out to: #g_gitaly
• Best individual to reach out to: @8bitlife

Expectations

What release does this feature occur in first?

13.7

### What are we expecting to happen?

Some performance improvements and less ruby code in codebase.

What might happen if this goes wrong?

Synchronisation of mirrors could fail.

What can we monitor to detect problems with this?

Logs and error ratio for FetchRemote RPC.

https://dashboards.gitlab.net/d/000000199/gitaly-feature-status?orgId=1&refresh=5m&from=now-12h&to=now&var-environment=gprd&var-method=FetchRemote&var-prometheus=prometheus-01-inf-gprd

Roll Out Steps

• Read the documentation of feature flags
• Add featureflagstaging to this issue
• Enable on staging
• Test on staging
• Ensure that documentation has been updated
• Announce on the 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
• Cross post chatops slack command to #support_gitlab-com and in your team channel
• Announce on the issue that the flag has been enabled
• Enable feature by default
• Remove feature flag and add changelog entry
• Close this issue