[Scheduler] Warning upon preemption and Swapping (#4647)

Co-authored-by: Robert Shaw <114415538+robertgshaw2-neuralmagic@users.noreply.github.com>

vllm/engine/llm_engine.py

@@ -737,6 +737,8 @@ class LLMEngine:
         num_generation_tokens_iter = 0
         time_to_first_tokens_iter: List[float] = []
         time_per_output_tokens_iter: List[float] = []
+        num_preemption_iter = (0 if scheduler_outputs is None else
+                               scheduler_outputs.preempted)
 
         # Request stats
         #   Latency
@@ -830,7 +832,6 @@ class LLMEngine:
 
         return Stats(
             now=now,
-
             # System stats
             #   Scheduler State
             num_running_sys=num_running_sys,
@@ -846,6 +847,7 @@ class LLMEngine:
             time_to_first_tokens_iter=time_to_first_tokens_iter,
             time_per_output_tokens_iter=time_per_output_tokens_iter,
             spec_decode_metrics=spec_decode_metrics,
+            num_preemption_iter=num_preemption_iter,
 
             # Request stats
             #   Latency