[Misc] Multi-turn benchmark output performance json (#39572)

Signed-off-by: NickLucche <nlucches@redhat.com>

benchmarks/multi_turn/benchmark_serving_multi_turn.py

@@ -1439,6 +1439,12 @@ async def main() -> None:
         action="store_true",
         help="Export summary to Excel file (optional)",
     )
+    parser.add_argument(
+        "--stats-json-output",
+        type=str,
+        default=None,
+        help="Export per-request stats (ttft_ms, tpot_ms, etc.) to a JSON file",
+    )
     parser.add_argument(
         "-v",
         "--verbose",
@@ -1651,6 +1657,19 @@ async def main() -> None:
         warmup_runtime_sec=warmup_runtime_sec,
     )
 
+    if args.stats_json_output is not None:
+        # Export per-request metrics as a JSON array for downstream analysis.
+        stats_data = [s._asdict() for s in client_metrics]
+        logger.info(
+            f"{Color.GREEN}Writing per-request stats JSON: "
+            f"{args.stats_json_output}{Color.RESET}"
+        )
+        os.makedirs(
+            os.path.dirname(os.path.abspath(args.stats_json_output)), exist_ok=True
+        )
+        with open(args.stats_json_output, "w") as f:
+            json.dump(stats_data, f, indent=2)
+
     if args.output_file is not None:
         # Write a JSON file with the updated conversations
         # The "assistant" content will contain the answers from the tested LLM