Adds regression test for and fixes metadata label syncing.

[rtibbles]

Summary

Description of the change(s) you made

• Adds support to the sync utility to sync the new metadata labels
• Adds a regression test to verify the fix

Manual verification steps performed

1. Wrote regression test.
2. Fixed behaviour!

References

Noticed in QA by @pcenov here: #3701 (comment)

Contributor's Checklist

Testing:

• Critical and brittle code paths are covered by unit tests

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Reviewer's Checklist

This section is for reviewers to fill out.

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md

[bjester]

@rtibbles @pcenov Referring to the comment on the former PR, is it clear that the option to sync "Titles and Descriptions" results in syncing these metadata attributes? Should we add a followup issue for updating the copy, or should we block this until we are able to do that?

[rtibbles]

An excellent question - I think given that we've not annotated any public channels with metadata yet, I think we can block this for now and retarget to unstable?

[bjester]

looks good. followup added to address UI concerns