langchain: preserve document metadata in FlashrankRerank (#19148)

**Description:** Preserves document metadata in `FlashrankRerank`
    - **Issue:** #19142
    - **Dependencies:** None
    - **Twitter handle:** n/a

---------

Co-authored-by: Simon Stone <simon.stone@dartmouth.edu>

docs/docs/integrations/retrievers/flashrank-reranker.ipynb

@@ -53,7 +53,10 @@
     "def pretty_print_docs(docs):\n",
     "    print(\n",
     "        f\"\\n{'-' * 100}\\n\".join(\n",
-    "            [f\"Document {i+1}:\\n\\n\" + d.page_content for i, d in enumerate(docs)]\n",
+    "            [\n",
+    "                f\"Document {i+1}:\\n\\n{d.page_content}\\nMetadata: {d.metadata}\"\n",
+    "                for i, d in enumerate(docs)\n",
+    "            ]\n",
     "        )\n",
     "    )"
    ]
@@ -316,6 +319,8 @@
     ").load()\n",
     "text_splitter = RecursiveCharacterTextSplitter(chunk_size=500, chunk_overlap=100)\n",
     "texts = text_splitter.split_documents(documents)\n",
+    "for idx, text in enumerate(texts):\n",
+    "    text.metadata[\"id\"] = idx\n",
     "\n",
     "embedding = OpenAIEmbeddings(model=\"text-embedding-ada-002\")\n",
     "retriever = FAISS.from_documents(texts, embedding).as_retriever(search_kwargs={\"k\": 20})\n",