<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def fetch(self, end_date=values.unset, minutes=values.unset, start_date=values.unset, task_channel=values.unset):
 """ Fetch a WorkersCumulativeStatisticsInstance :param datetime end_date: Filter cumulative statistics by a end date. :param unicode minutes: Filter cumulative statistics by up to 'x' minutes in the past. :param datetime start_date: Filter cumulative statistics by a start date. :param unicode task_channel: Filter cumulative statistics by TaskChannel. :returns: Fetched WorkersCumulativeStatisticsInstance :rtype: twilio.rest.taskrouter.v1.workspace.worker.workers_cumulative_statistics.WorkersCumulativeStatisticsInstance """

        params = values.of({
            'EndDate': serialize.iso8601_datetime(end_date),
            'Minutes': minutes,
            'StartDate': serialize.iso8601_datetime(start_date),
            'TaskChannel': task_channel,
        })

        payload = self._version.fetch(
            'GET',
            self._uri,
            params=params,
        )

        return WorkersCumulativeStatisticsInstance(
            self._version,
            payload,
            workspace_sid=self._solution['workspace_sid'],
        )