Backport of "Expose opt-in tagger dates in ListRefs" to 19.4

What does this MR do and why?

Backports !9318 (merged) (commit bda869a710901b6493af83d1da8f88f2cd506e14) to this stable branch.

This Gitaly change should be backported to 19.3 and 19.4 as a prerequisite for backporting the Rails Tags API performance fix. This adds opt-in tagger timestamps to ListRefs, allowing Rails to preserve created_at without per-tag lookups.

Compatibility and backport details

• Adds include_tagger_date and Reference.tagger_date; date retrieval is disabled by default. Enabling it may require additional object reads.
• Preserves missing dates, epoch zero, nested-tag timestamps, and existing overflow behavior.
• Retains the stable branch's v18 Go module paths when resolving the cherry-pick conflict.
• The original MR merged on October 8 and first deployed to GitLab.com at 2026-10-08T18:40:39.920Z.
• The incident fix requires the dependent Rails change to ship with this Gitaly support.

References

• Performance bug: https://gitlab.com/gitlab-org/gitlab/-/issues/630430
• Rails migration: gitlab#614152
• Gitaly prerequisite: #7352

Validation

• Passed on both backports: full ./internal/gitaly/service/ref test suite with GITALY_TESTING_GIT_BINARY=/opt/homebrew/bin/git.
• Passed: regenerated ref.pb.go matches the backport using protoc 30.2 and each branch's recorded Go generator version; protolint passes for proto/ref.proto.
• Passed: formatting and whitespace checks; independent backport review found no blockers.
• CI is running: https://gitlab.com/gitlab-org/gitaly/-/pipelines/2937265508

How to set up and validate locally

1. Prepare a repository with annotated, lightweight, and nested tags with distinct timestamps.
2. Using this branch's server and client bindings, call ListRefs with include_tagger_date disabled, then enabled.
3. Confirm annotated and nested tags return their own timestamps; lightweight tags have no timestamp. Repeat with peel_tags: true.
4. Follow pagination cursors and confirm dates remain attached to the same refs.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
  • Backports a merged, SRE-requested prerequisite for the dependent Rails bug fix.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
  • The prerequisite MR's deployment is confirmed above.
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (if applicable)
  • If this MR fixes a bug that affects customers, the customer label has been applied.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the E2E Omnibus GitLab EE pipeline (triggered by the e2e:test-on-omnibus-ee job) has succeeded. If it has failed (or shows warnings), investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, request help in the #s_developer_experience Slack channel to confirm the failures are unrelated to the merge request.
  • Applies to the dependent Rails backport; Gitaly CI is pending separately.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).
• Once the backport has been merged, the commit changes will be automatically deployed to a release environment that can be used for manual validation. See after merging runbook for details.