Public API for getting metrics that have been set

I have a usecase where I want to get access to one or all the metrics that have been set with `putMetric` before sending a response back to the user. Specifically I'm planning on putting some of the metric values into [server timing headers](https://www.w3.org/TR/server-timing/) so that I can tie clientside behavior back to serverside metrics.

I'm found that while there is not a public `getMetric` method I **can** get access to previously set metrics by inspecting the data [here](https://github.com/awslabs/aws-embedded-metrics-node/blob/730bbaeb341ee3952a404c04fc1de59a6ec65cf5/src/logger/MetricsContext.ts#L37)

My question is whether this is an okay, somewhat stable method to use? Or if this is exclusively private/internal data that I should not be touching? If the latter, is there interest in adding formal `getMetric` or `getProperty` methods?

Start with src/logger/MetricsContext.ts at the referenced data and trace how putMetric stores values. Determine the supported public contract for retrieving one or all previously set metrics before a response is sent, including the distinction between public and internal data; done means the API decision and expected retrieval behavior are clear.

Written by the indexing model from the issue text.