fix: metrics_writeback takes three positional arguments

[greut]

Hey, running on 2.1.0 I'm hitting that when activating the prometheus metrics wrapper.

Cheers,

ERROR:elastalert:Traceback (most recent call last):
  File "/usr/local/lib/python3.9/site-packages/elastalert/elastalert.py", line 1506, in alert
    return self.send_alert(matches, rule, alert_time=alert_time, retried=retried)
  File "/usr/local/lib/python3.9/site-packages/elastalert/elastalert.py", line 1615, in send_alert
    res = self.writeback('elastalert', alert_body, rule)
TypeError: metrics_writeback() takes 3 positional arguments but 4 were given

[jertel]

Thanks for the contribution!