[Feature Flag] Roll out merge-tree merge

Enable git-merge-tree merges for UserMergeBranch

Enable the :gitaly_merge_tree_merge flag

Owners

• Team: @gl-gitaly
• Most appropriate slack channel to reach out to: #g_gitaly
• Best individual to reach out to: @jcaigitlab

Expectations

What release does this feature occur in first?

15.6

What are we expecting to happen?

Merges work as usual

What might happen if this goes wrong?

Merges will break

What can we monitor to detect problems with this?

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

Roll Out Steps

• Enable on staging
  • Is the required code deployed on staging? (howto)
  • Enable on staging (howto)
  • Add featureflagstaging to this issue (howto)
  • Test on staging (howto)
  • Verify the feature flag was used by checking Prometheus metric gitaly_feature_flag_checks_total
• Enable on production
  • Is the required code deployed on production? (howto)
  • Enable on production in #production (howto)
  • Add featureflagproduction to this issue
  • Verify the feature flag was used by checking Prometheus metric gitaly_feature_flag_checks_total
• Default-enable the feature flag (optional, only required if backwards-compatibility concerns exist)
  • Wait for release containg default-disabled feature flag.
  • Change the feature flag to default-enabled (howto)
  • Wait for release containing default-enabled feature flag.
• Remove feature flag
  • Remove the feature flag and the pre-feature-flag code (howto)
  • Remove the feature flag via chatops (howto)
  • Close this issue

Please refer to the documentation of feature flags for further information.