Remove the feature flag - 'gitaly_distributed_reads' - distribution of read operations among gitalies

Current Status

Scheduling the removal of the feature flag for %13.10. The feature flag is defaulted to on, so leaving the flag in the code has no functional impact.

What

Enabling of :gitaly_distributed_reads feature flag.

Owners

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

Expectations

What release does this feature occur in first?

13.3

### What are we expecting to happen?

The read operations should be distributed among up to date gitaly storages (the generation of the repository is the latest and the same across the main part of the fleet). This allows to reduce load on the primary gitaly node and enable secondaries to serve the actual request, not only operate on replication event and transactions support.

What might happen if this goes wrong?

• The praefect could return outdated data for read operations.
• Higher load on the Postgres database instance.

What can we monitor to detect problems with this?

The praefect staging overview page: SLO,

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
• Remove feature flag and add changelog entry
• Close this issue