community: change metadata in opensearch mmr (#30921)

- [ ] **PR message**:
- **Description:** including metadata_field in
max_marginal_relevance_search() would result in error, changed the logic
to be similar to how it's handled in similarity_search, where it can be
any field or simply a "*" to include every field

libs/community/langchain_community/vectorstores/opensearch_vector_search.py

@@ -1289,7 +1289,12 @@ class OpenSearchVectorSearch(VectorStore):
         return [
             Document(
                 page_content=results[i]["_source"][text_field],
-                metadata=results[i]["_source"][metadata_field],
+                metadata=(
+                    results[i]["_source"]
+                    if metadata_field == "*"
+                    or metadata_field not in results[i]["_source"]
+                    else results[i]["_source"][metadata_field]
+                ),
                 id=results[i]["_id"],
             )
             for i in mmr_selected