Import the metadata rule

What does this MR do and why?

This introduces a few mr_ methods on helper and imports the metadata rule from https://gitlab.com/gitlab-org/gitlab/-/blob/master/danger/z_metadata/Dangerfile.

MR acceptance checklist

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

• I have evaluated the MR acceptance checklist for this MR.

Closes #11 (closed).