Update MongoDBChatMessageHistory to create an index on SessionId (#5632)

All the queries to the database are done based on the SessionId
property, this will optimize how Mongo retrieves all messages from a
session

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Tag maintainers/contributors who might be interested:
@dev2049

langchain/memory/chat_message_histories/mongodb.py

@@ -47,6 +47,7 @@ class MongoDBChatMessageHistory(BaseChatMessageHistory):
 
         self.db = self.client[database_name]
         self.collection = self.db[collection_name]
+        self.collection.create_index("SessionId")
 
     @property
     def messages(self) -> List[BaseMessage]:  # type: ignore