[bench] add start_times field to vllm bench serve json result (#32667)

Signed-off-by: Kebe <mail@kebe7jun.com>

vllm/benchmarks/serve.py

@@ -958,6 +958,7 @@ async def benchmark(
             "output_lens": actual_output_lens,
             "ttfts": [output.ttft for output in outputs],
             "itls": [output.itl for output in outputs],
+            "start_times": [output.start_time for output in outputs],
             "generated_texts": [output.generated_text for output in outputs],
             "errors": [output.error for output in outputs],
             "max_output_tokens_per_s": metrics.max_output_tokens_per_s,
@@ -1721,6 +1722,7 @@ async def main_async(args: argparse.Namespace) -> dict[str, Any]:
         for field in [
             "input_lens",
             "output_lens",
+            "start_times",
             "ttfts",
             "itls",
             "generated_texts",