[metric spec review] MetricReader implements non-specified ForceFlush

[dyladan]

MetricReader doesn't have ForceFlush according to the SDK spec.

opentelemetry-js/experimental/packages/opentelemetry-sdk-metrics/src/export/MetricReader.ts

Line 171 in d154066

async forceFlush(options?: ForceFlushOptions): Promise<void> {

Originally posted by @reyang in open-telemetry/community#1204 (comment)

[dyladan]

MetricReader doesn't have ForceFlush according to the SDK spec.

open-telemetry/opentelemetry-js@d154066/experimental/packages/opentelemetry-sdk-metrics/src/export/MetricReader.ts#L171

Hmm, correct. 🤔
However, it is referenced in the spec for the MeterProvider.

ForceFlush MUST invoke ForceFlush on all registered MetricReader and Push Metric Exporter instances.

Both the Python and Java SDK implementations seem to have solved it in the same way. May be worth opening a spec issue over if one does not yet exist. 🤔

originally posted by @pichlermarc

[pichlermarc]

Adding to above comment:

I think this is just something that is missing in the specification, I have created an issue (open-telemetry/opentelemetry-specification#2924) to clarify.

IMO this should not block the release as there is a precedent of other SDKs implementing it in the same way as we do.

[dyladan]

Given the spec wording that exists and other implementations, I think we're ok to close this. @legendecas @pichlermarc @reyang feel free to reopen or comment if you disagree