add unconstrained buffer and load shed metrics

[jhrldev]

This introduces visibility into Load Shed occurrences and Buffer Queue usage/capacity.

For now this is a draft because tests are missing.

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Checklist

Complete the checklist (and note appropriate exceptions) before the PR is marked ready-for-review.

• PR description explains the motivation for the change and relevant context for reviewing
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• Changeset is included for user-facing changes
• Changes are compatible¹
• Documentation² completed
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• Metrics and logs are added³ and documented
• Tests added and passing⁴
  • Unit tests
  • Integration tests
  • Manual tests, as necessary

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4. Tick whichever testing boxes are applicable. If you are adding Manual Tests, please document the manual testing (extensively) in the Exceptions. ↩

[apollo-librarian]

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[github-actions]

@jhrldev, please consider creating a changeset entry in /.changesets/. These instructions describe the process and tooling.