Fix corrupted JSONL metrics file due to concurrent writes (#19011)

python/sglang/srt/managers/request_metrics_exporter.py

@@ -87,6 +87,7 @@ class FileRequestMetricsExporter(RequestMetricsExporter):
 
         # File handler state management
         self._current_file_handler = None
+        self._current_file_lock = asyncio.Lock()
         self._current_hour_suffix = None
 
     def _ensure_file_handler(self, hour_suffix: str):
@@ -135,6 +136,7 @@ class FileRequestMetricsExporter(RequestMetricsExporter):
             current_time = datetime.now()
             hour_suffix = current_time.strftime("%Y%m%d_%H")
 
+            async with self._current_file_lock:
                 # Ensure correct file handler is open for current hour
                 self._ensure_file_handler(hour_suffix)