langchain/retrievers/vespa_retriever.py/VespaRetriever/__init__
class VespaRetriever:    def __init__(
        self,
        app: Vespa,
        body: Dict,
        content_field: str,
        metadata_fields: Optional[Sequence[str]] = None,
    ):
        self._application = app
        self._query_body = body
        self._content_field = content_field
        self._metadata_fields = metadata_fields or ()