Write debug instead of exception after FormulaEngine stop

Replace exception with debug message to avoid confusing traceback output.
Previously, FormulaEngine couldn't be stopped, so exception was acceptable.
Now that FormulaEngine can be stopped, use severity level 'debug' since
stopping is a normal operation and usefull only during debugging.

Author: ela-kotulska-frequenz

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

@@ -313,7 +313,7 @@ async def _run(self) -> None:
             try:
                 msg = await evaluator.apply()
             except asyncio.CancelledError:
-                _logger.exception("FormulaEngine task cancelled: %s", self._name)
+                _logger.debug("FormulaEngine task cancelled: %s", self._name)
                 raise
             except Exception as err:  # pylint: disable=broad-except
                 _logger.warning(
@@ -582,7 +582,7 @@ async def _run(self) -> None:
                     phase_3.value,
                 )
             except asyncio.CancelledError:
-                _logger.exception("FormulaEngine task cancelled: %s", self._name)
+                _logger.debug("FormulaEngine task cancelled: %s", self._name)
                 break
             else:
                 await sender.send(msg)