FirestoreChatMessageHistory add timestamp

[SaschaHeyer]

Hi Google Team

the FirestoreChatMessageHistory currently is not storing a timestamp.
Adding a timestamp would allow us to use Firestores TTL feature.
This will allow us to clean up all chat histories older than X days.

Useful for multiple reasons:

1. remove chat histories that are no longer active anymore
2. reduce costs on the Firestore side
3. reduce costs on the Gemini side e.g smaller context size

what do you think?

Best regards