Verify compliant metric SDK specification implementation: Defaults and configuration

[MrAlias]

• Identify all the normative requirements, recommendations, and options the specification defines as comments to this issue
• Ensure the current metric SDK implementation is compliant with these normative requirements, recommendations, and options in those comments.

[pellared]

The SDK MUST provide configuration according to the SDK environment variables specification.

From https://github.com/open-telemetry/opentelemetry-specification/blob/v1.21.0/specification/configuration/sdk-environment-variables.md:

SDKs MAY choose to allow configuration via the environment variables in this specification, but are not required to.

Therefore, the most important thing is to check if there are no environment variable is marked as stable which is not defined in https://github.com/open-telemetry/opentelemetry-specification/blob/v1.21.0/specification/configuration/sdk-environment-variables.md and if the defined env vars are implemented as specified.

Following metrics related env vars are defined:

• OTEL_METRICS_EXPORTER (Stable) - not implemented, but can be later implemented in https://pkg.go.dev/go.opentelemetry.io/contrib/exporters/autoexport
• OTEL_METRICS_EXEMPLAR_FILTER (Experimental?) - not implemented. But I do not think it is required. More: Add support for exemplars #559
• OTEL_METRIC_EXPORT_INTERVAL (Stable) - implemented in periodic reader as defined per spec
• OTEL_METRIC_EXPORT_TIMEOUT (Stable) - implemented in periodic reader as defined per spec

I do not find any env vars that would make https://pkg.go.dev/go.opentelemetry.io/otel/sdk/metric@v0.39.0 not compliant with the specification.

[pellared]

It looks like we comply with this fragment of the specification.

@MrAlias Can you please double-check and close if you agree with my verification?

[MrAlias]

LGTM