code: function getAgentByUrl (requestUrl) {
    if (!requestUrl) {
      return null;
    }
    var parsedRequestUrl = url.parse(requestUrl);
    var proxyUrl = getSystemProxyByUrl(parsedRequestUrl);
    if (!proxyUrl) {
      log.debug('No system proxy');
      return null;
    }
  
    log.debug('Detect system proxy Url: ' + proxyUrl);
    var parsedProxyUrl = url.parse(proxyUrl);
    var tunnelOptions = getTunnelOptionsByProxy(parsedProxyUrl, parsedRequestUrl);
    if (tunnelOptions) {
      var tunnelFnName = getTunnelFnName(parsedProxyUrl, parsedRequestUrl);
      log.debug('Tunnel name is ' + tunnelFnName);
      return tunnel[tunnelFnName](tunnelOptions);
    }
  }
docs: Get agent only if it has system proxy