Proposal: Clear InMemoryMetricExporter Data Post-Shutdown to Align with Other Exporters

[paper2]

• This only affects the JavaScript OpenTelemetry library
• This may affect other libraries, but I would like to get opinions here first

Proposal

The InMemoryMetricExporter currently allows users to retrieve metrics through getMetrics even after the shutdown method has been called. To improve consistency with other in-memory exporters (such as InMemorySpanExporter and InMemoryLogRecordExporter), it may be beneficial to clear the metrics array after shutdown, preventing further retrieval.

If this proposal is accepted, I would be interested in implementing the changes myself :)

Background

Metric Exporter Specification

According to the specification, allowing getMetrics to return values after shutdown is technically compliant.

Shuts down the exporter. Called when SDK is shut down. This is an opportunity for exporter to do any cleanup required.

Shutdown SHOULD be called only once for each MetricExporter instance. After the call to Shutdown subsequent calls to Export are not allowed and should return a Failure result.

Shutdown SHOULD NOT block indefinitely (e.g., if it attempts to flush the data and the destination is unavailable). OpenTelemetry SDK authors MAY decide if they want to make the shutdown timeout configurable.

However, based on the line:

This is an opportunity for exporter to do any cleanup required.

it seems more intuitive that the getMetrics function should return a cleared array after shutdown, in line with expected behavior for cleanup.

Current Implementation of InMemoryMetricExporter

Currently, InMemoryMetricExporter does not clear the array returned by getMetrics at shutdown. As a result, users can still retrieve data with getMetrics even after shutdown.

opentelemetry-js/packages/sdk-metrics/src/export/InMemoryMetricExporter.ts

Lines 77 to 80 in 4b5c21c

	shutdown(): Promise<void> {
	this._shutdown = true;
	return Promise.resolve();
	}

Implementation in InMemorySpanExporter and InMemoryLogRecordExporter

In contrast, InMemorySpanExporter does clean up its data after shutdown, clearing the array returned by getFinishedSpans.

opentelemetry-js/packages/opentelemetry-sdk-trace-base/src/export/InMemorySpanExporter.ts

Lines 48 to 52 in 4b5c21c

	shutdown(): Promise<void> {
	this._stopped = true;
	this._finishedSpans = [];
	return this.forceFlush();
	}

Similarly, InMemoryLogRecordExporter also resets its internal state at shutdown.

opentelemetry-js/experimental/packages/sdk-logs/src/export/InMemoryLogRecordExporter.ts

Lines 52 to 56 in 4b5c21c

	public shutdown(): Promise<void> {
	this._stopped = true;
	this.reset();
	return Promise.resolve();
	}

This inconsistency in behavior among different in-memory exporters could lead to confusion for users (as it did for me).

[dyladan]

I don't think the difference here is intentional