Refactor InMemoryMetricExporter and add tests

[nstawski]

Description

This is a pull to separate the refactor of InMemoryMetricExporter from its usage in the pull#3486

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Added tests for InMemoryMetricExporter

Does This PR Require a Contrib Repo Change?

• No.

Checklist:

• Followed the style guidelines of this project
• Unit tests have been added

[nstawski]

Don't think this pull needs the changelog update, but happy to add it if necessary.

[pmcollins]

LGTM with one suggestion.

[pmcollins]

Suggested change

	be retrieved using the `metrics` attribute.
	be retrieved using the `_counter` attribute.

[pmcollins]

If you did intend to highlight the metrics attribute, you could rephrase the text to something like:

This class is designed for testing purposes. It maintains exported metrics in the metrics dictionary attribute.

[srikanthccv]

I don't think we need to add InMemoryMetricExporter to the public API. The InMemoryMetricReader is what should be used for testing. What problem does InMemoryMetricExporter solve that InMemoryMetricReader can't do?

[ocelotl]

I don't think we need to add InMemoryMetricExporter to the public API. The InMemoryMetricReader is what should be used for testing. What problem does InMemoryMetricExporter solve that InMemoryMetricReader can't do?

I agree, @nstawski, I'm closing this PR because of the reasons above, please let me know if you disagree 👍