exposure method
Implementation
@override
Future<void> exposure(
String flagkey, ExperimentVariant? variant, String instanceName) async {
final properties = {'variant': variant?.value, 'flag_key': flagkey};
if (variant == null) {
properties.remove('variant');
}
final event = {'event_type': "\$exposure", 'event_properties': properties};
await _channel
.invokeMethod('track', {'instanceName': instanceName, 'event': event});
await _channel.invokeMethod('flush', {'instanceName': instanceName});
}