fix(metrics): Clear custom dimensions when flushing.

[phipag]

Summary

This PR clears custom dimensions when .flush() is called on the metrics instance. Note: This does not affect calls to .flushMetrics() since they create a temporary second metrics instance that supports flushing on its own with its own dimension set (copy).

This was the default behavior in v1 of Powertools Java but is also the same behavior in the other runtimes such as TypeScript (see comment).

On flush, we clear:

• Custom dimensions and preserve default dimensions
• Metadata (also consistent with TS runtime)

Changes

Issue number: #2327

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[surajkumar]

I have tested this code with the original snippet from the bug and now no metrics are being emitted. In the log group I am seeing:

{
    "level": "WARN",
    "message": "No metrics were emitted",
    "function_arn": null,
    "function_name": null,
    "function_request_id": null,
    "function_version": null,
    "service": null,
    "timestamp": "2025-12-10T13:24:42.385Z"
}

To add context on how I am pulling the changes. I have grabbed a copy of the EmfMetricsLogger and create the instance like so:

private static final Metrics METRICS = new EmfMetricsLogger(new EnvironmentProvider(), new MetricsContext());

Don't think this makes a difference though for this small test. Deploying to mavenLocal might be a bit more tricky for me.

[phipag]

@surajkumar Thanks for testing! can you try to call as last statement in your testing code METRICS.flush()?

The reason why you see this warning is because @FlushMetrics uses a request-scoped proxy to a thread-local metrics instance (https://github.com/aws-powertools/powertools-lambda-java/blob/main/powertools-metrics/src/main/java/software/amazon/lambda/powertools/metrics/MetricsFactory.java#L33). Hence, @FlushMetrics emitted this warning because it did not collect any metrics. It created an instance which was not used because you manually created a separate EmfMetricsLogger.

[surajkumar]

@surajkumar Thanks for testing! can you try to call as last statement in your testing code METRICS.flush()?

The reason why you see this warning is because @FlushMetrics uses a request-scoped proxy to a thread-local metrics instance (https://github.com/aws-powertools/powertools-lambda-java/blob/main/powertools-metrics/src/main/java/software/amazon/lambda/powertools/metrics/MetricsFactory.java#L33). Hence, @FlushMetrics emitted this warning because it did not collect any metrics. It created an instance which was not used because you manually created a separate EmfMetricsLogger.

I tried with the flush at the end of the method and yes that works.

I then re-tried with MetricsFactory.getMetricsInstance() (again, copying it over and updating where needed) and it gives me the same result of "No metrics were emitted" - Going to pull the repo, build and test - I'll get back to you.

[surajkumar]

Pulled repo and performed my manual integration test. All looks good to me. Thanks @phipag !

[phipag]

Awesome, thanks for confirming @surajkumar!

Merged this PR.