Add documents and embbedings database metrics to stats response

[Alirexaa]

Pull Request

Related issue

Fixes #650
Fixes #649

What does this PR do?

This pull request includes an update to the IndexStats struct in the src/indexes.rs file to add several new fields for more detailed tracking of index statistics.

Changes in IndexStats struct:

• Added raw_document_db_size field to track the size of the raw document database.
• Added avg_document_size field to track the average size of documents.
• Added number_of_embedded_documents field to track the number of embedded documents.
• Added number_of_embeddings field to track the number of embeddings.

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!