@Override protected void doCancel() {
    CompletableFuture<AggregatedHttpResponse> responseFuture = this.responseFuture;
    if (responseFuture != null) {
      responseFuture.cancel(false);
    }
  }
Returns the {@linkplain HttpHeaders headers} for this request.
zipkin-storage/elasticsearch/src/main/java/zipkin2/elasticsearch/internal/client/HttpCall.java