[Metrics SDK] Add InMemory metric exporter

[lalitb]

As required by the specs - https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/sdk_exporters/in-memory.md

This would also be great tool for testing.

[marcalff]

Also, please consider the visitor idea, detailed in:

#1698

[github-actions]

This issue was marked as stale due to lack of activity.