How to unit test decorator registrations using SimpleInjector 3+?

Follow up to an older question here. Say I have a registration like the following:

container.Register(typeof(IHandleCommand<>), _handlerAssemblies, Lifestyle.Transient);
container.RegisterDecorator(typeof(IHandleCommand<>),
  typeof(MetricsCommandHandlerWrapper<>), Lifestyle.Singleton);

Where the MetricsCommandHandlerWrapper is defined like so:

public class MetricsCommandHandlerWrapper<T> : IHandleCommand<T> where T: ICommand
{
    private readonly ICollectMetrics _metrics;
    private readonly Func<IHandleCommand<T>> _handlerFactory;

    public MetricsCommandHandlerWrapper(ICollectMetrics metrics,
      Func<IHandleCommand<T>> handlerFactory)
    {
      _metrics = metrics;
      _handlerFactory = handlerFactory;
    }

    public async Task HandleAsync(T command)
    {
        // code to record metrics around command handling which eventually invokes
        await _handlerFactory().HandleAsync(command).ConfigureAwait(false);
    }
}

How can I write a unit test that asserts the actual decoratee handlers are registered with Transient lifestyle?

I have tried composing the root and inspecting the registration for a closed IHandleCommand<FakeCommand> type, which reveals an ImplementationType of MetricsCommandHandlerWrapper<FakeCommand> as expected. Invoking GetRelationships() on that registration reveals its 2 dependencies, ICollectMetrics and the one I am interested in, the Func<IHandleCommand<FakeCommand>> factory delegate, which is registered as a Singleton. However invoking .Dependency.GetInstance() on that factory delegate throws an exception that the instance producer returned null.

Is there any way to assert that the inner decoratee is registered as Transient, and if so, how?