[Bugfix] Fix E2EL mean and median stats (vllm-project#9984)

Signed-off-by: daitran2k1 <tranquangdai7a@gmail.com>
Signed-off-by: Sumit Dubey <sumit.dubey2@ibm.com>

Author: daitran2k1

benchmarks/benchmark_serving.py

@@ -406,9 +406,9 @@ def calculate_metrics(
         median_itl_ms=np.median(itls or 0) * 1000,
         percentiles_itl_ms=[(p, np.percentile(itls or 0, p) * 1000)
                             for p in selected_percentiles],
-        mean_e2el_ms=np.median(e2els or 0) * 1000,
+        mean_e2el_ms=np.mean(e2els or 0) * 1000,
         std_e2el_ms=np.std(e2els or 0) * 1000,
-        median_e2el_ms=np.mean(e2els or 0) * 1000,
+        median_e2el_ms=np.median(e2els or 0) * 1000,
         percentiles_e2el_ms=[(p, np.percentile(e2els or 0, p) * 1000)
                              for p in selected_percentiles],
     )