feat: added element metadata to unstructured loader (#1068)

### Summary

Adds tracked metadata from `unstructured` elements to the document
metadata when `UnstructuredFileLoader` is used in `"elements"` mode.
Tracked metadata is available in `unstructured>=0.4.9`, but the code is
written for backward compatibility with older `unstructured` versions.

### Testing

Before running, make sure to upgrade to `unstructured==0.4.9`. In the
code snippet below, you should see `page_number`, `filename`, and
`category` in the metadata for each document. `doc[0]` should have
`page_number: 1` and `doc[-1]` should have `page_number: 2`. The example
document is `layout-parser-paper-fast.pdf` from the [`unstructured`
sample
docs](https://github.com/Unstructured-IO/unstructured/tree/main/example-docs).

```python
from langchain.document_loaders import UnstructuredFileLoader
loader = UnstructuredFileLoader(file_path=f"layout-parser-paper-fast.pdf", mode="elements")
docs = loader.load()
```

langchain/document_loaders/unstructured.py

@@ -33,12 +33,19 @@ class UnstructuredFileLoader(BaseLoader):
     def load(self) -> List[Document]:
         """Load file."""
         elements = self._get_elements()
-        metadata = {"source": self.file_path}
         if self.mode == "elements":
-            docs = [
-                Document(page_content=str(el), metadata=metadata) for el in elements
-            ]
+            docs: List[Document] = list()
+            for element in elements:
+                metadata = {"source": self.file_path}
+                # NOTE(MthwRobinson) - the attribute check is for backward compatibility
+                # with unstructured<0.4.9. The metadata attributed was added in 0.4.9.
+                if hasattr(element, "metadata"):
+                    metadata.update(element.metadata.to_dict())
+                if hasattr(element, "category"):
+                    metadata["category"] = element.category
+                docs.append(Document(page_content=str(element), metadata=metadata))
         elif self.mode == "single":
+            metadata = {"source": self.file_path}
             text = "\n\n".join([str(el) for el in elements])
             docs = [Document(page_content=text, metadata=metadata)]
         else: