std::string source = "http://localhost:8084/geoserver/wfs?";
osg::ref_ptr<WFSFeatureSource> wfsSource = new WFSFeatureSource;
wfsSource->setURL(osgEarth::URI(source));
wfsSource->setTypeName("countries");
wfsSource->setOutputFormat("GML2");
//wfsSource->setOutputFormat("json");
auto count = wfsSource->getFeatureCount();
std::cout << count; // result:-1

osg::ref_ptr<FeatureModelLayer> wfsLayer = new FeatureModelLayer();
wfsLayer->setName("wfsLayer");
wfsLayer->setFeatureSource(wfsSource);
map->addLayer(wfsLayer);