@SuppressWarnings("FutureReturnValueIgnored")
  // TODO: errorprone wants us to check this future before returning, but what would be a sensible
  // check? Say it is somehow canceled, would we take action? Would callback.onError() be redundant?
  @Override protected void doEnqueue(Callback<V> callback) {
    sendRequest().handle((response, t) -> {
      if (t != null) {
        callback.onError(t);
      } else {
        try {
          V value = parseResponse(response, bodyConverter);
          callback.onSuccess(value);
        } catch (Throwable t1) {
          propagateIfFatal(t1);
          callback.onError(t1);
        }
      }
      return null;
    });
  }
Returns the {@linkplain HttpHeaders headers} for this request.
zipkin-storage/elasticsearch/src/main/java/zipkin2/elasticsearch/internal/client/HttpCall.java