Backport of 'Add FF to toggle namespace filtering for Duo Chat data'

What does this MR do and why?

Backports feature flag introduced with !216709 (merged) to 18.7.

With !210409 (merged) we accidentally removed a feature flag that toggles namespace path filtering for legacy Duo chat events on some GraphQL endpoints like aiUserMetrics. Because legacy Duo chat data still does not have namespace_path property backfilled, the change did brake DAP related reports for some customers. !216709 (merged) reintroduces the feature flag only for legacy Duo chat data.

For more information, check this thread: #582944 (comment 2954497363)

References

• #582944 (closed)
• !216709 (merged)

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.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• 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
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  • 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:test-on-omnibus-ee job has succeeded, or if it has failed, investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, reach out to a Software Engineer in Test in #s_developer_experience.

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