我有一个测试，我试图收集活动/跨度在内存中使用内存导出器。但测试失败，异常显示集合为空。
我尝试在Assert之前添加延迟，但结果仍然相同。设置中是否还需要其他内容，以便开始捕获活动？

[Fact]
    public void Can_collect_Activities_in_memory()
    {
        var serviceName = "ServiceName";
        var activities = new List<Activity>();

        var tracerProvider = Sdk.CreateTracerProviderBuilder()
            .AddSource(serviceName)
            .SetResourceBuilder(ResourceBuilder.CreateDefault().AddService("in-memory"))
            .AddInMemoryExporter(activities)
            .Build();

        var source = new ActivitySource(serviceName);

        var activity = source.StartActivity("TestActivity");

        Assert.Single(activities);
    }

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