Counter constructor
Counter({})
Construct a new Counter with a name, help text and optional
labelNames.
If labelNames are provided, use labels(...) to assign label values.
The optional collectCallback is called at the beginning of collect and
allows to update the value of the counter before collecting it.
Implementation
Counter({
required String name,
required String help,
List<String> labelNames = const [],
this.collectCallback,
}) : super(name: name, help: help, labelNames: labelNames);