Filtering on nested metadata not allowed

[jadendick]

When trying to filter by metadata that contains nested JSON objects, the field name is rejected since it contains '.'. For example:

Metadata: {"common": {"source": "confluence"}}
Filter: {"common.source": "confluence"}
This does not work because of the '.' which is currently not allowed but is a valid JsonPath.

I believe this can be fixed by changing the check here, perhaps to a simple regex function that allows '.':

langchain-postgres/langchain_postgres/vectorstores.py

Line 103 in 6d4f6f3

if _classes is not None:

Thank you