fix: BigQueryVectorSearch JSON type unsupported for metadatas
diff --git a/libs/community/langchain_community/vectorstores/bigquery_vector_search.py b/libs/community/langchain_community/vectorstores/bigquery_vector_search.py
index 3c77ffb917ea4e..da89dc34c2b2f0 100644
--- a/libs/community/langchain_community/vectorstores/bigquery_vector_search.py
+++ b/libs/community/langchain_community/vectorstores/bigquery_vector_search.py
@@ -404,7 +404,8 @@ def get_documents(
             if self.metadata_field:
                 metadata = row[self.metadata_field]
             if metadata:
-                metadata = json.loads(metadata)
+                if not isinstance(metadata, dict):
+                    metadata = json.loads(metadata)
             else:
                 metadata = {}
             metadata["__id"] = row[self.doc_id_field]
@@ -544,7 +545,8 @@ def _search_with_score_and_embeddings_by_vector(
         for row in job:
             metadata = row[self.metadata_field]
             if metadata:
-                metadata = json.loads(metadata)
+                if not isinstance(metadata, dict):
+                    metadata = json.loads(metadata)
             else:
                 metadata = {}
             metadata["__id"] = row[self.doc_id_field]

Thank you for contributing to LangChain!

- [x] **PR title**: "community: fix error in BigQueryVectorSearch"

- [x] **PR message**: 
    - **Description:** Fixes the incorrect parsing of the metadata column
    - **Issue:** The metadata column can be of type JSON or STRING [1] but the code only works for the STRING type.

- [x] **Add tests and docs**: If you're adding a new integration, please include
  1. An example notebook: [demo](https://github.com/ashleyxuu/langchain/blob/master/docs/docs/integrations/vectorstores/bigquery_vector_search.ipynb)

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