Per-experiment sampling?

When using the `science` method in the `Scientist` module, I want to supply an extra parameter that would be used inside the `enabled?` method to determine whether to run the experiment.

The intention is to be able to control, on a per-experiment basis, the percentage of time that the experiment will actually run.

Is something like this possible?

首先阅读 Scientist 模块中的 `science` 方法和 `enabled?` 方法，以了解如何选择要执行的实验。在实现之前定义每个实验的采样行为及其 API；完成时应包括覆盖率，以证明配置的百分比控制着实验是否运行。

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