Facilitate subclassing of MetricsLogger

[ajpower]

Context

Our team wants to subclass MetricsLogger to track which metrics properties are being set. However, we are facing two issues that are preventing us from doing so:

1. The context property is private. This requires us to manually track the properties that have been set through setProperty.
2. MetricsLogger's constructor requires parameters whose types are defined in internal modules, which makes it difficult to define a constructor of our own in the subclass.

Ask

Our request is to make MetricsLogger more amenable to subclassing. Specifically, we would like to

1. Make MetricsLogger.context a protected property
2. Export resolveEnvironment and MetricsContext in the index.js as part of aws-embedded-metrics-node's public API.

I am happy to submit a PR if our request is approved.