feat: implement flush in write_api

Author: minsungoh

influxdb_client/client/write_api.py

@@ -387,8 +387,8 @@ def write_payload(payload):
 
     def flush(self):
         """Flush data."""
-        # TODO
-        pass
+        if self._subject:
+            self._subject.on_completed()
 
     def close(self):
         """Flush data and dispose a batching buffer."""
@@ -411,8 +411,8 @@ def __exit__(self, exc_type, exc_val, exc_tb):
 
     def __del__(self):
         """Close WriteApi."""
+        self.flush()
         if self._subject:
-            self._subject.on_completed()
             self._subject.dispose()
             self._subject = None