fix: worker logs with extra json_fields

[michaelkedar]

was getting errors like this:

>>> logging.info('hello', extra={'json_fields': {'hi': 'there'}})
Traceback (most recent call last):
  File "<python-input-17>", line 1, in <module>
    logging.info('hello', extra={'json_fields': {'hi': 'there'}})
    ~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.13/logging/__init__.py", line 2196, in info
    root.info(msg, *args, **kwargs)
    ~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.13/logging/__init__.py", line 1520, in info
    self._log(INFO, msg, args, **kwargs)
    ~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.13/logging/__init__.py", line 1663, in _log
    record = self.makeRecord(self.name, level, fn, lno, msg, args,
                             exc_info, func, extra, sinfo)
  File "/usr/lib/python3.13/logging/__init__.py", line 1637, in makeRecord
    raise KeyError("Attempt to overwrite %r in LogRecord" % key)
KeyError: "Attempt to overwrite 'json_fields' in LogRecord"

Changing the extra information injector from a factory to a filter seems to fix this