Roll out feature flag deployment_filters

What

In !44041 (merged) we introduce the feature flag deployment_filters. This feature flag is disabled by default. We need to roll this out, then remove it once deemed stable.

Owners

• Team: Delivery
• Most appropriate slack channel to reach out to: #g_delivery
• Best individual to reach out to: @yorickpeterse

Expectations

What are we expecting to happen?

The filter will be available without issues

What might happen if this goes wrong?

Filtering by merge requests with one of the new filters may be slow or time out.

What can we monitor to detect problems with this?

There is no easy way to monitor the exact filters using Grafana. Fortunately, we can test this quite easily: just use the filters for gitlab-org/gitlab. This project is big enough that any performance issues that may be present will surface quickly.

Roll Out Steps

• Enable on staging (/chatops run feature set deployment_filters true --staging)
• Test on staging
• Coordinate a time to enable the flag with #production and #g_delivery on slack.
• Announce on the issue an estimated time this will be enabled on GitLab.com
• Enable on GitLab.com by running chatops command in #production (/chatops run feature set deployment_filters true)
• Cross post chatops Slack command to #support_gitlab-com (more guidance when this is necessary in the dev docs) and in your team channel
• Announce on the issue that the flag has been enabled
• Remove feature flag and add changelog entry
• Remove MergeRequestsFinder#by_deployment method
• Ensure that documentation has been updated: !46343 (merged)
• After the flag removal is deployed, clean up the feature flag by running chatops command in #production channel