[Metrics] Log number of preempted requests (#28522)

Add tracking and periodic logging for the number of preempted requests in the
metrics logger. This helps monitor system behavior under load.
Signed-off-by: Yining Liu <610lyn@gmail.com>

vllm/v1/metrics/loggers.py

@@ -118,12 +118,14 @@ class LoggingStatLogger(StatLoggerBase):
         self.num_prompt_tokens: int = 0
         self.num_generation_tokens: int = 0
         self.num_corrupted_reqs: int = 0
+        self.num_preemptions: int = 0
 
     def _track_iteration_stats(self, iteration_stats: IterationStats):
         # Save tracked stats for token counters.
         self.num_prompt_tokens += iteration_stats.num_prompt_tokens
         self.num_generation_tokens += iteration_stats.num_generation_tokens
         self.num_corrupted_reqs += iteration_stats.num_corrupted_reqs
+        self.num_preemptions += iteration_stats.num_preempted_reqs
 
     def _get_throughput(self, tracked_stats: int, now: float) -> float:
         # Compute summary metrics for tracked stats
@@ -196,17 +198,30 @@ class LoggingStatLogger(StatLoggerBase):
             "Avg generation throughput: %.1f tokens/s",
             "Running: %d reqs",
             "Waiting: %d reqs",
-            "GPU KV cache usage: %.1f%%",
-            "Prefix cache hit rate: %.1f%%",
         ]
         log_args = [
             self.last_prompt_throughput,
             self.last_generation_throughput,
             self.last_scheduler_stats.num_running_reqs,
             self.last_scheduler_stats.num_waiting_reqs,
+        ]
+
+        if self.num_preemptions > 0:
+            log_parts.append("Preemptions: %d")
+            log_args.append(self.num_preemptions)
+
+        log_parts.extend(
+            [
+                "GPU KV cache usage: %.1f%%",
+                "Prefix cache hit rate: %.1f%%",
+            ]
+        )
+        log_args.extend(
+            [
                 self.last_scheduler_stats.kv_cache_usage * 100,
                 self.prefix_caching_metrics.hit_rate * 100,
             ]
+        )
 
         if envs.VLLM_COMPUTE_NANS_IN_LOGITS:
             log_parts.append("Corrupted: %d reqs")