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Describe the bug
When we use the database session service for short term memory, it is not storing the custoim metadata field, Hence every a2a request is going like a new requesting and the A2A server is not able to recognize that and treating every request as a new request.
like we are doing it here
https://github.com/google/adk-python/blob/main/src/google/adk/sessions/vertex_ai_session_service.py#L403

To Reproduce
Steps to reproduce the behavior:
Try to spin up an a2a server and test with the database session service by asking follow up questions and see that the metadata is not getting stored in the database.