Starts the task thread.

def start(self):
        """
        Starts the task thread.
        """
        self._lock.acquire()
        try:
            if not self._thread:
                self._thread = Thread(target=self._target)
                self._thread.setDaemon(True)
                self._thread.name = "elasticapm sender thread"
                self._thread.start()
        finally:
            self._lock.release()