Make some fields in *Stats structs optional to support older servers

These three fields were added in 1.13. If you want to support older
servers, these fields need to be optional.

https://github.com/meilisearch/meilisearch/releases/tag/v1.13.0

Author: LukasKalbertodt

src/client.rs

@@ -1152,8 +1152,9 @@ pub struct ClientStats {
     /// Storage space claimed by Meilisearch and LMDB in bytes
     pub database_size: usize,
 
-    /// Storage space used by the database in bytes, excluding unused space claimed by LMDB
-    pub used_database_size: usize,
+    /// Storage space used by the database in bytes, excluding unused space claimed by LMDB.
+    /// Is `None` for Meilisearch servers older than 1.13.
+    pub used_database_size: Option<usize>,
 
     /// When the last update was made to the database in the `RFC 3339` format
     #[serde(with = "time::serde::rfc3339::option")]

src/indexes.rs

@@ -1900,10 +1900,12 @@ pub struct IndexStats {
     pub number_of_documents: usize,
 
     /// Total number of documents with at least one embedding
-    pub number_of_embedded_documents: usize,
+    /// Is `None` for Meilisearch servers older than 1.13.
+    pub number_of_embedded_documents: Option<usize>,
 
     /// Total number of embeddings in an index
-    pub number_of_embeddings: usize,
+    /// Is `None` for Meilisearch servers older than 1.13.
+    pub number_of_embeddings: Option<usize>,
 
     /// Storage space claimed by all documents in the index in bytes
     pub raw_document_db_size: usize,