Create metric SDK testing utilities for instrumentation

[MrAlias]

Similar to the go.opentelemetry.io/otel/sdk/trace/tracetest package, testing utilities need to be created for the metric SDK.

Blocks:

• Add integration testing to the host instrumenation opentelemetry-go-contrib#2755
• Add integration testing to the runtime instrumenation opentelemetry-go-contrib#2757
• Add back metric integration testing to the otelhttp/test package opentelemetry-go-contrib#2759
• Add back metric integration testing to the otelgocql/test package opentelemetry-go-contrib#2761

Proposal

• Provide a Reader implementation that allows instrumentation to easily assert the expected metric data was collected. The metricdatatest package already provides exported data comparison, the Reader would only need to facilitate collection and transfer of that data.

[MrAlias]

This might be as simple as having instrumentation use a manual reader.

[MrAlias]

Verified, the manual reader can be used as a functional testing utility alongside the metricdatatest package: open-telemetry/opentelemetry-go-contrib@092a97c

No need for an additional package.

[bmon]

Verified, the manual reader can be used as a functional testing utility alongside the metricdatatest package: open-telemetry/opentelemetry-go-contrib@092a97c

Thank you very much for this link, I hadn't quite joined the dots here, the example test is really helpful!