message: Add rationale explaining why we do the header modifications in the filter
diff: @@ -102,6 +102,11 @@ export function buildMiddleware(
  // Use the custom middleware filter to do two things:
  // 1. Remove any headers not in the allow list to stop them being forwarded
  // 2. Only permit the allowed HTTP methods if configured
  + //
  + // We are filtering the proxy request headers here rather than in
  + // `onProxyReq` becuase when global-agent is enabled then `onProxyReq`
  + // fires _after_ the agent has already sent the headers to the proxy
  + // target, causing a ERR_HTTP_HEADERS_SENT crash
  const filter = (_pathname: string, req: http.IncomingMessage): boolean => {
  const headerNames = Object.keys(req.headers);
  headerNames.forEach(h => {
  