Add documents database metrics to indexStats type

[consoleLogIt]

Pull Request

Related issue

Fixes #1875

What does this PR do?

adds the documents database metrics to indexStats type.

• Modify the stats API to include indexes.INDEXNAME.rawDocumentDbSize
• Modify the stats API to include indexes.INDEXNAME.avgDocumentSize

PR checklist

Please check if your PR fulfills the following requirements:

• Does this PR fix an existing issue, or have you listed the changes applied in the PR description (and why they are needed)?
• Have you read the contributing guidelines?
• Have you made sure that the title is accurate and descriptive of the changes?

Thank you so much for contributing to Meilisearch!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.84%. Comparing base (0a0217e) to head (ad9939c).
Report is 90 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1889   +/-   ##
=======================================
  Coverage   98.84%   98.84%           
=======================================
  Files          17       17           
  Lines        1565     1565           
  Branches      338      338           
=======================================
  Hits         1547     1547           
  Misses         18       18           

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[flevi29]

bors merge

[flevi29]

@consoleLogIt Thanks again!

[meili-bors]

Build succeeded:

• integration-tests (Node.js 22)
• types-check
• style-check
• integration-tests (Node.js 18)
• integration-tests (Node.js 20)