onHeaders method
Implementation
void onHeaders(Service service, Map<String, String> headers) {
print('Received header for ${service.name} metadata: $headers');
// Merge metadata.
headers.forEach((key, value) {
if (key.toLowerCase() != 'trailer' &&
key.toLowerCase() != 'content-type') {
metadata[key] = value;
}
});
service.connected = true;
onopen?.call(service);
}