How to access upstream server from responseheaders event?

help
1 
from .mechanism import StatManager
import mitmproxy


class StatAddon:

    def responseheaders(self, flow: mitmproxy.http.HTTPFlow):
        """
            HTTP response headers were successfully read. At this point,
            the body
            is empty.
        """
        StatManager(
            flow.response.headers["host"],
            flow.response.status_code,
            # third parameter is proxy used, how to access it from here ?
        )```