[Core] Avoid metrics log noise when idle (#8868)

Signed-off-by: Russell Bryant <rbryant@redhat.com>

vllm/engine/metrics.py

@@ -421,6 +421,11 @@ def get_throughput(tracked_stats: List[int], now: float,
 class LoggingStatLogger(StatLoggerBase):
     """LoggingStatLogger is used in LLMEngine to log to Stdout."""
 
+    def __init__(self, *args, **kwargs) -> None:
+        super().__init__(*args, **kwargs)
+        self.last_prompt_throughput: Optional[float] = None
+        self.last_generation_throughput: Optional[float] = None
+
     def log(self, stats: Stats) -> None:
         """Called by LLMEngine.
            Logs to Stdout every self.local_interval seconds."""
@@ -445,8 +450,14 @@ class LoggingStatLogger(StatLoggerBase):
                 now=stats.now,
                 last_log=self.last_local_log)
 
-            # Log to stdout.
-            logger.info(
+            log_fn = logger.info
+            if not any((prompt_throughput, generation_throughput,
+                        self.last_prompt_throughput,
+                        self.last_generation_throughput)):
+                # Avoid log noise on an idle production system
+                log_fn = logger.debug
+
+            log_fn(
                 "Avg prompt throughput: %.1f tokens/s, "
                 "Avg generation throughput: %.1f tokens/s, "
                 "Running: %d reqs, Swapped: %d reqs, "
@@ -472,11 +483,16 @@ class LoggingStatLogger(StatLoggerBase):
                     self._format_spec_decode_metrics_str(
                         self.spec_decode_metrics))
 
+            self._reset(stats, prompt_throughput, generation_throughput)
+
+    def _reset(self, stats, prompt_throughput, generation_throughput) -> None:
         # Reset tracked stats for next interval.
         self.num_prompt_tokens = []
         self.num_generation_tokens = []
         self.last_local_log = stats.now
         self.spec_decode_metrics = None
+        self.last_prompt_throughput = prompt_throughput
+        self.last_generation_throughput = generation_throughput
 
     def _format_spec_decode_metrics_str(
             self, metrics: "SpecDecodeWorkerMetrics") -> str: