Adds usedDatabaseSize key to stats

[pratts]

Pull Request

Related issue

Fixes #613

What does this PR do?

• Adds usedDatabaseSize key to stats

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 86.15%. Comparing base (e56172a) to head (e2b7493).
Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #621   +/-   ##
=======================================
  Coverage   86.15%   86.15%           
=======================================
  Files          14       14           
  Lines        2854     2854           
=======================================
  Hits         2459     2459           
  Misses        283      283           
  Partials      112      112           

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[curquiza]

bors merge

[meili-bors]

Build succeeded:

• integration-tests (go current version)
• integration-tests (go latest version)
• linter