@Override
    public void subscribe(Subscriber<? super Connection> actual) {
        actual.onSubscribe(new StateMachineSubscription(actual));
    }
Design notes:
- ConnectionPublisher is Mono-like (0..1), the request amount is ignored
- given the testing nature, the performance requirements are less strict
- "synchronized" is used to avoid races
- Reactive Streams spec violations are not checked (e.g. non-positive request)
modules/r2dbc/src/main/java/org/testcontainers/r2dbc/ConnectionPublisher.java