[Feature flag] Enable gpg_signing feature flag

What

Enable the :gitaly_gpg_signing feature flag. This flag enables Gitaly to use the new gitaly-gpg binary to sign commits.

Owners

• Team: 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?

16.2

What are we expecting to happen?

Nothing on gitlab.com, since we don't have the signing_key configured. But on self service customers, commit signing should work if the feature flag is turned on.

What might happen if this goes wrong?

commit signing ends up signing commits incorrectly.

What can we monitor to detect problems with this?

1. Open https://dashboards.gitlab.net/d/000000199/gitaly-feature-status
2. Change "method" to UserMergeBranch
3. Copy/paste the URL & change gprd to gstd to monitor staging as well as prod

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)
  • Progressively enable 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.