func setPeerURLsHeader(req *http.Request, urls types.URLs) {
	if urls == nil {
		// often not set in unit tests
		return
	}
	peerURLs := make([]string, urls.Len())
	for i := range urls {
		peerURLs[i] = urls[i].String()
	}
	req.Header.Set("X-PeerURLs", strings.Join(peerURLs, ","))
}
func setPeerURLsHeader(req *http.Request, urls types.URLs) {
	if urls == nil {
		// often not set in unit tests
		return
	}
	peerURLs := make([]string, urls.Len())
	for i := range urls {
		peerURLs[i] = urls[i].String()
	}
	req.Header.Set("X-PeerURLs", strings.Join(peerURLs, ","))
}
// setPeerURLsHeader reports local urls for peer discovery