internal HttpRequestTaskCallable(HttpClient httpClient, IHttpUriRequest request,
                                  HttpContext context, ResponseHandler <V> responseHandler, FutureCallback <V> callback
                                  , FutureRequestExecutionMetrics metrics)
 {
     this.httpclient      = httpClient;
     this.responseHandler = responseHandler;
     this.request         = request;
     this.context         = context;
     this.callback        = callback;
     this.metrics         = metrics;
 }