mitmproxy: add a `My-Counter` header incremented with every request

add_header.py

from mitmproxy import http

class AddHeader:
    def __init__(self):
        self.counter = 0

    def request(self, flow: http.HTTPFlow) -> None:
        self.counter += 1
        flow.request.headers["My-Counter"] = str(self.counter)

addons = [
    AddHeader()
]