langchain/retrievers/time_weighted_retriever.py/TimeWeightedVectorStoreRetriever/aadd_documents
class TimeWeightedVectorStoreRetriever:    async def aadd_documents(
        self, documents: List[Document], **kwargs: Any
    ) -> List[str]:
        """Add documents to vectorstore."""
        current_time = kwargs.get("current_time")
        if current_time is None:
            current_time = datetime.datetime.now()
        # Avoid mutating input documents
        dup_docs = [deepcopy(d) for d in documents]
        for i, doc in enumerate(dup_docs):
            if "last_accessed_at" not in doc.metadata:
                doc.metadata["last_accessed_at"] = current_time
            if "created_at" not in doc.metadata:
                doc.metadata["created_at"] = current_time
            doc.metadata["buffer_idx"] = len(self.memory_stream) + i
        self.memory_stream.extend(dup_docs)
        return await self.vectorstore.aadd_documents(dup_docs, **kwargs)