collect method
Implementation
@override
Future<Iterable<MetricFamilySamples>> collect() async {
await collectCallback?.call(this);
final samples = <Sample>[];
_children.forEach((labelValues, child) =>
samples.add(Sample(name, labelNames, labelValues, child._value)));
return [MetricFamilySamples(name, MetricType.counter, help, samples)];
}