subscribe method
Registers a listener that will be called every time ping is invoked.
Throws an AssertionError if the Pingora has already been disposed.
Implementation
void subscribe(VoidCallback listener) {
assert(
!_disposed,
AssertionError('Can not subscribe to a disposed Pingora'),
);
_listeners.add(listener);
developer.log(
'Pingora<$runtimeType> subscribed (listeners: ${_listeners.length})',
name: 'pingora',
);
}