Keycloak fails to start with KC_METRICS_ENABLED=true — NoClassDefFoundError: io/opentelemetry/common/impl/ApiUsageLogger

[naman7898]

Hi,
Keycloak 26.6 fails to start when metrics are enabled (KC_METRICS_ENABLED=true)

Steps to reproduce

docker run --rm -it -p 8080:8080 \
  -e KC_BOOTSTRAP_ADMIN_USERNAME=admin \
  -e KC_BOOTSTRAP_ADMIN_PASSWORD=admin \
  -e KC_METRICS_ENABLED=true \
  dhi.io/keycloak:26.6 start-dev

Expected Behavior

Keycloak starts successfully with metrics enabled.

Actual Behavior

Keycloak fails to start with the following error:

Updating the configuration and installing your custom providers, if any. Please wait.
2026-05-18 11:27:04,088 INFO  [io.quarkus.deployment.QuarkusAugmentor] (main) Quarkus augmentation completed in 3079ms
Running the server in development mode. DO NOT use this configuration in production.
2026-05-18 11:27:05,532 INFO  [org.infinispan.CONTAINER] (main) ISPN000974: Virtual threads support: enabled
2026-05-18 11:27:05,985 INFO  [org.hibernate.orm.jdbc.batch] (JPA Startup Thread) HHH100501: Automatic JDBC statement batching enabled (maximum batch size 32)
2026-05-18 11:27:06,008 INFO  [com.arjuna.ats.jbossatx] (main) ARJUNA032014: Stopping transaction recovery manager
2026-05-18 11:27:06,019 ERROR [io.vertx.core.impl.TaskQueue] (vert.x-internal-blocking-1) Caught unexpected Throwable: java.lang.NoClassDefFoundError: io/opentelemetry/common/impl/ApiUsageLogger
	at io.opentelemetry.api.trace.Span.fromContextOrNull(Span.java:60)
	at io.quarkus.micrometer.runtime.export.exemplars.OpentelemetryExemplarSamplerProvider$1.get(OpentelemetryExemplarSamplerProvider.java:36)
	at io.quarkus.micrometer.runtime.export.exemplars.OpentelemetryExemplarSamplerProvider$1.isSampled(OpentelemetryExemplarSamplerProvider.java:32)
	at io.prometheus.client.exemplars.DefaultExemplarSampler.doSample(DefaultExemplarSampler.java:46)
	at io.prometheus.client.exemplars.DefaultExemplarSampler.sample(DefaultExemplarSampler.java:34)
	at io.micrometer.prometheus.PrometheusTimer.updateLastExemplar(PrometheusTimer.java:128)
	at io.micrometer.prometheus.PrometheusTimer.recordNonNegative(PrometheusTimer.java:117)
	at io.micrometer.core.instrument.AbstractTimer.record(AbstractTimer.java:260)
	at io.micrometer.core.instrument.composite.CompositeTimer.record(CompositeTimer.java:54)
	at io.quarkus.micrometer.runtime.binder.vertx.EventTiming.end(EventTiming.java:19)
	at io.quarkus.micrometer.runtime.binder.vertx.VertxPoolMetrics.begin(VertxPoolMetrics.java:120)
	at io.quarkus.micrometer.runtime.binder.vertx.VertxPoolMetrics.begin(VertxPoolMetrics.java:18)
	at io.vertx.core.impl.WorkerTask.run(WorkerTask.java:52)
	at io.vertx.core.impl.TaskQueue.run(TaskQueue.java:81)
	at org.jboss.threads.ContextHandler$1.runWith(ContextHandler.java:18)
	at org.jboss.threads.EnhancedQueueExecutor$Task.doRunWith(EnhancedQueueExecutor.java:2651)
	at org.jboss.threads.EnhancedQueueExecutor$Task.run(EnhancedQueueExecutor.java:2630)
	at org.jboss.threads.EnhancedQueueExecutor$ThreadBody.run(EnhancedQueueExecutor.java:1586)
	at org.jboss.threads.DelegatingRunnable.run(DelegatingRunnable.java:11)
	at org.jboss.threads.ThreadLocalResettingRunnable.run(ThreadLocalResettingRunnable.java:11)
	at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
	at java.base/java.lang.Thread.run(Unknown Source)
Caused by: java.lang.ClassNotFoundException: io.opentelemetry.common.impl.ApiUsageLogger
	... 22 more

2026-05-18 11:27:07,021 WARN  [io.quarkus.vertx.core.runtime.VertxCoreRecorder] (main) Vert.x shutdown timed out after 1000ms
2026-05-18 11:27:07,033 ERROR [org.keycloak.quarkus.runtime.cli.ExecutionExceptionHandler] (main) ERROR: Failed to start server in (development) mode
2026-05-18 11:27:07,034 ERROR [org.keycloak.quarkus.runtime.cli.ExecutionExceptionHandler] (main) ERROR: io/opentelemetry/common/impl/ApiUsageLogger
2026-05-18 11:27:07,034 ERROR [org.keycloak.quarkus.runtime.cli.ExecutionExceptionHandler] (main) ERROR: io.opentelemetry.common.impl.ApiUsageLogger
2026-05-18 11:27:07,034 ERROR [org.keycloak.quarkus.runtime.cli.ExecutionExceptionHandler] (main) For more details run the same command passing the '--verbose' option. Also you can use '--help' to see the details about the usage of the particular command.

Can we please look into this issue and have this resolved. Thanks