Close resampled data receivers when closing formula engines

Signed-off-by: Sahas Subramanian <[email protected]>

Author: shsms

src/frequenz/sdk/timeseries/formula_engine/_formula_engine.py

@@ -117,6 +117,9 @@ async def _stop(self) -> None:
         if self._task is None:
             return
         await cancel_and_await(self._task)
+        _, fetchers = self._builder.finalize()
+        for fetcher in fetchers.values():
+            fetcher.stream.close()
 
     @classmethod
     def from_receiver(