[Benchmark] Enable benchmark to run with encoding_format="bytes"

[DarkLight1337]

Purpose

Support benchmarking #27066

Test Plan

vllm bench serve \
    --model Qwen/Qwen3-Embedding-0.6B \
    --backend openai-embeddings \
    --endpoint /v1/embeddings \
    --dataset-name sharegpt \
    --dataset-path benchmarks/ShareGPT_V3_unfiltered_cleaned_split.json \
    --extra_body '{"encoding_format": "bytes"}'

Test Result

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• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

[gemini-code-assist]

Code Review

This pull request adds support for encoding_format="bytes" in the benchmarking tool for pooling requests. The change correctly handles the case where metadata is sent in response headers instead of the body. I've added one comment to improve the robustness of header parsing by using .get() to avoid potential KeyErrors and provide more informative error messages on failure. Overall, the change is good and addresses the intended purpose.

[gemini-code-assist]

Using direct dictionary access response.headers["metadata"] is unsafe as it will raise a KeyError if the header is missing. While the surrounding try...except block will catch this, the resulting error message ('metadata') is not very informative for debugging. It's more robust to use .get() to safely access the header and raise a ValueError with a clear error message if it's not present. This will improve error reporting for failed benchmark requests.

metadata_str = response.headers.get("metadata")
if not metadata_str:
    raise ValueError("Missing 'metadata' header for 'bytes' encoding.")
metadata = json.loads(metadata_str)
usage = metadata.get("usage", {})