var consumer = await system.CreateConsumer(
    new ConsumerConfig
    {
        Reference = Guid.NewGuid().ToString(),
        Stream = stream,
        OffsetSpec = new OffsetTypeFirst(),
        MessageHandler = async (consumer, ctx, message) =>
        {
            Console.WriteLine($"message: {Encoding.Default.GetString(message.Data.Contents.ToArray())} - consumed");
            await Task.CompletedTask;
        }
    });