Metric test helpers needed

[jmacd]

This repo has a copy of the trace testing helpers from the main repository. This repo does not have a copy of the metric testing helpers. We should add test helpers to support testing metrics in this repository, but first some code should be exposed from the main repository (e.g., AsyncInstrumentState).

[matej-g]

Would take this together with #385

[MrAlias]

Closing, stale. A manual reader and the metricdatatest package are used to test expected metric exports.