Add source metadata to bedrock retriever response (#21349)

Thank you for contributing to LangChain!

- [X] **PR title**: "community: Add source metadata to bedrock retriever
response"

- [X] **PR message**: 
- **Description:** Bedrock retrieve API returns extra metadata in the
response which is currently not returned in the retriever response
- **Issue:** The change adds the metadata from bedrock retrieve API
response to the bedrock retriever in a backward compatible way. Renamed
metadata to sourceMetadata as metadata term is being used in the
Document already. This is in sync with what we are doing in llama-index
as well.
    - **Dependencies:** No


- [X] **Add tests and docs**:
  1. Added unit tests
  2. Notebook already exists and does not need any change
3. Response from end to end testing, just to ensure backward
compatibility: `[Document(page_content='Exoplanets.',
metadata={'location': {'s3Location': {'uri':
's3://bucket/file_name.txt'}, 'type': 'S3'}, 'score': 0.46886647,
'source_metadata': {'x-amz-bedrock-kb-source-uri':
's3://bucket/file_name.txt', 'tag': 'space', 'team': 'Nasa', 'year':
1946.0}})]`


- [X] **Lint and test**: Run `make format`, `make lint` and `make test`
from the root of the package(s) you've modified. See contribution
guidelines for more: https://python.langchain.com/docs/contributing/

Additional guidelines:
- Make sure optional dependencies are imported within a function.
- Please do not add dependencies to pyproject.toml files (even optional
ones) unless they are required for unit tests.
- Most PRs should not touch more than one package.
- Changes should be backwards compatible.
- If you are adding something to community, do not re-import it in
langchain.

If no one reviews your PR within a few days, please @-mention one of
baskaryan, efriis, eyurtsev, hwchase17.

---------

Co-authored-by: Piyush Jain <[email protected]>

libs/community/langchain_community/retrievers/bedrock.py

@@ -115,13 +115,16 @@ def _get_relevant_documents(
         results = response["retrievalResults"]
         documents = []
         for result in results:
+            content = result["content"]["text"]
+            result.pop("content")
+            if "score" not in result:
+                result["score"] = 0
+            if "metadata" in result:
+                result["source_metadata"] = result.pop("metadata")
             documents.append(
                 Document(
-                    page_content=result["content"]["text"],
-                    metadata={
-                        "location": result["location"],
-                        "score": result["score"] if "score" in result else 0,
-                    },
+                    page_content=content,
+                    metadata=result,
                 )
             )