Verify compliant metric SDK specification implementation: MetricReader/MetricReader operations/Shutdown

[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.

[dashpole]

Shutdown

opentelemetry-go/sdk/metric/reader.go

Lines 91 to 103 in cbc5890

	// Shutdown flushes all metric measurements held in an export pipeline and releases any
	// held computational resources.
	//
	// This deadline or cancellation of the passed context are honored. An appropriate
	// error will be returned in these situations. There is no guaranteed that all
	// telemetry be flushed or all resources have been released in these
	// situations.
	//
	// After Shutdown is called, calls to Collect will perform no operation and instead will return
	// an error indicating the shutdown state.
	//
	// This method needs to be concurrent safe.
	Shutdown(context.Context) error

Shutdown MUST be called only once for each MetricReader instance. After the call to Shutdown, subsequent invocations to Collect are not allowed. SDKs SHOULD return some failure for these calls, if possible.

In Shutdown(), readers store a shutdownProducer in the produer, which returns an error that is returned to the caller of Collect:

opentelemetry-go/sdk/metric/manual_reader.go

Lines 107 to 110 in cbc5890

	// Any future call to Collect will now return ErrReaderShutdown.
	mr.sdkProducer.Store(produceHolder{
	produce: shutdownProducer{}.produce,
	})

opentelemetry-go/sdk/metric/periodic_reader.go

Lines 330 to 333 in cbc5890

	// Any future call to Collect will now return ErrReaderShutdown.
	ph := r.sdkProducer.Swap(produceHolder{
	produce: shutdownProducer{}.produce,
	})

Shutdown SHOULD provide a way to let the caller know whether it succeeded, failed or timed out.

Shutdown returns an error.

Shutdown SHOULD complete or abort within some timeout.

Shutdown does not impose a timeout on collection or exporting of telemetry in the periodic reader

It does accept a context, which allows users to easily place a timeout on collection or export.