Flakyness inside the QuarkusTests when asserting a gauge Value in a meterRegistry

[fbecar22]

Describe the bug

Hi 🌞 Switching to quarkus-micrometer-opentelemetry extension, I found no bugs in the normal usage of the extension (amazing with LGTM devservice 🤩 ) but I have a strange behaviour in "@QuarkusTest" when trying to assert gauge values in a MeterRegistry.
I made a lot tests but I always have random successes and fails asserting a gauge value in a meterRegistry.

Expected behavior

No Flakyness inside the QuarkusTests when asserting a gauge Value in a meterRegistry.

Actual behavior

Flaky tests

How to Reproduce?

Here is a repository where I reproduced this issue : https://github.com/fbecar22/otel-micrometer-quarkustest-bug

The flaky test is located here : https://github.com/fbecar22/otel-micrometer-quarkustest-bug/blob/main/src/test/java/fr/fbecar22/MetricsRegistrationTest.java

Output of uname -a or ver

Darwin FRL-C02GX0N8Q6LW 24.4.0 Darwin Kernel Version 24.4.0: Fri Apr 11 18:32:43 PDT 2025; root:xnu-11417.101.15~117/RELEASE_ARM64_T8103 arm64

Output of java -version

21

Quarkus version or git rev

3.22.3<

Build tool (ie. output of mvnw --version or gradlew --version)

mvn 3.9.8

Additional information

No response

[quarkus-bot]

/cc @brunobat (micrometer), @ebullient (micrometer)

[brunobat]

Interesting. Can you please rename the issue to something like "Flakyness inside the QuarkusTests when asserting a gauge Value in a meterRegistry", as the current title is a bit misleading.

[brunobat]

This is not a bug but it might require documentation enhancements.

There are a few things wrong with the reproducer:

• You were registering the count.items gauge twice. Once on startup and another one when running the test.
• When you add quarkus.micrometer.export.json.enabled=true in application.properties you are adding a new Micrometer registry, apart from the one implemented with OTel. When asserting, you were getting sometimes the json registry, sometimes the OTel one. Spoiler: The OTel one is not returning a value, therefore the NaN.
• You should not inject either the MM registry or the OTel exporter from the test classloader. You should Isolate the call with a rest call. See: https://github.com/quarkusio/quarkus/blob/ebee0a4b66d1b4dba9c51165e78d22fbf3b90d3e/integration-tests/micrometer-prometheus/src/main/java/io/quarkus/ExporterResource.java

Saying this, the MM registry implemented with OTel is not exposing the metrics data. For that you need to look at the output of the OpenTelemetry exporter. You should follow these steps:

1. add OTel testing dependency:

  <dependency>
      <groupId>io.opentelemetry</groupId>
      <artifactId>opentelemetry-sdk-testing</artifactId>
  </dependency>

2. In the runtime code you must override our default OTel exporter by an in-memory one: it can be inside your ItemMetrics class for symplicity:

 @ApplicationScoped
    static class InMemoryMetricExporterProducer {
        @Produces
        @Singleton
        InMemoryMetricExporter inMemoryMetricsExporter() {
            return InMemoryMetricExporter.create();
        }
    }

3. Inject the exporter in the REST endpoint:

@Inject
    InMemoryMetricExporter inMemoryMetricExporter;

4. Expose the data on a REST endpoint:

@GET
public Long getMeterValue() {
    return Double.valueOf(inMemoryMetricExporter.getFinishedMetricItems().stream()
                    .filter(metricData -> metricData.getName().equals("count.items"))
                    .map(metricData -> metricData.getDoubleGaugeData()
                            .getPoints().iterator().next()
                            .getValue())
                    .findAny().orElse(0d))
            .longValue();
}

However, it's API is not as straight forward, as you can see.

Please let me know if I can close the issue.

[fbecar22]

@brunobat A bit late but thanks a lot for these explanations. 🙏
It helps us a lot in the understanding of the opentelemetry integration in quarkus! crystal clear !

I close the issue as it does not block us anymore!

[brunobat]

Thanks for the issue, I've improved the docs: https://quarkus.io/version/main/guides/opentelemetry#using-cdi-to-produce-a-test-exporter