_monitoring.py: Ensure buffer gets flushed in non-async mode

buildgrid/server/_monitoring.py

@@ -156,9 +156,11 @@ class MonitoringBus:
                 output_writers.append(output_file)
 
                 while True:
-                    if await __streaming_worker(iter(output_file)):
+                    if await __streaming_worker([output_file]):
                         self.__sequence_number += 1
 
+                        output_file.flush()
+
             else:
                 output_writers.append(sys.stdout.buffer)