Recently, I have been working with Google ADK, and I have noticed that there is not any function for creating session with MongoDB. Currently, there are only InMemorySessionService, VertexAiSessionService and DatabaseSessionService. This last service is only available for relational database connection.
MongoDB handles a document type of key values, so there are possibilities to include these types for tracking and managing conversations threads with ADK, as InMemorySessionService functionality core.