Missing step to uncomment dotenv configuration

We should mention somewhere around [here](https://www.apollographql.com/docs/tutorial/production.html#api-key) that [this line](https://github.com/apollographql/fullstack-tutorial/blob/master/final/server/src/index.js#L1) should be uncommented in order for `dotenv` configuration to take effect. Without this, metrics are not sent to Apollo Engine since the Apollo server does not actually have the ENGINE_API_KEY set. Also, some acknowledgement of how this works via the `dotenv` module seems like it would be appropriate.

从生产教程中 API 密钥部分附近开始，检查第 1 行的 server/src/index.js，其中引用了 dotenv 配置。更新文档，解释如何取消该行的注释，以及 dotenv 如何提供 ENGINE_API_KEY 以发送 Apollo Engine 指标；教程应明确说明这一设置。

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