[v1.13] Add documents database metrics to stats

Following the changes related to Meilisearch v1.13.0 mega issue:

Description

Update the stats object to include the new documents database metrics.

For further reference, see the related Meilisearch engine issue.

Specifications

• [ ] Modify the stats API to include indexes.INDEXNAME.rawDocumentDbSize in SDK responses
• [ ] Modify the stats API to include indexes.INDEXNAME.avgDocumentSize in SDK responses