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Quickstart/Node.js Metrics: Add Node.js Metrics Codelab

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Codelab for Node.js metrics quickstart @odeke-em please review!

This [page ](https://opencensus.io/advanced-concepts/context-propagation/) has no content, would it be better to add some filler content using these: - https://github.com/census-instrumentation/opencensus-specs/blob/master/trace/HTTP.md#propagation - https://github.com/census-instrumentation/opencensus-specs#context as a starting point @odeke-em ? Additionally, we could...