Protobuf version incompatibility with `OtlpMeterRegistry` in newly-generated Micronaut projects since Micronaut 4.9.0

[jon-signal]

Expected Behavior

Attempting to inject a MeterRegistry with OtlpMeterRegistry on the classpath in an otherwise-minimal project should work without issue.

Actual Behaviour

Attempting to inject a MeterRegistry with OtlpMeterRegistry on the classpath in an otherwise-minimal project results in the following exception:

io.micronaut.context.exceptions.BeanInstantiationException: 
Error instantiating bean of type  [io.micrometer.core.instrument.composite.CompositeMeterRegistry]

Message: com/google/protobuf/RuntimeVersion$RuntimeDomain
Path Taken:
c.e.OtlpMeterRegistryDependencyDemoTest#meterRegistry
\---> @i.m.c.a.Primary @j.i.Singleton i.m.c.i.c.CompositeMeterRegistry i.m.c.m.m.MeterRegistryFactory.compositeMeterRegistry#compositeMeterRegistry([List<MeterRegistry> registries], List<MeterRegistryConfigurer<MeterRegistry>> configurers)
	at io.micronaut.context.DefaultBeanContext.resolveByBeanFactory(DefaultBeanContext.java:2357)
	at io.micronaut.context.DefaultBeanContext.createRegistration(DefaultBeanContext.java:3162)
	at io.micronaut.context.SingletonScope.getOrCreate(SingletonScope.java:80)
	at io.micronaut.context.DefaultBeanContext.resolveBeanRegistration(DefaultBeanContext.java:3012)
	at io.micronaut.context.DefaultBeanContext.addCandidateToList(DefaultBeanContext.java:3625)
	at io.micronaut.context.DefaultBeanContext.resolveBeanRegistrations(DefaultBeanContext.java:3580)
	at io.micronaut.context.DefaultBeanContext.getBeanRegistrations(DefaultBeanContext.java:3554)
	at io.micronaut.context.DefaultBeanContext.getBeansOfType(DefaultBeanContext.java:1480)
	at io.micronaut.context.AbstractBeanResolutionContext.getBeansOfType(AbstractBeanResolutionContext.java:240)
	at io.micronaut.context.AbstractInitializableBeanDefinition.resolveBeansOfType(AbstractInitializableBeanDefinition.java:2226)
	at io.micronaut.context.AbstractInitializableBeanDefinition.getBeansOfTypeForConstructorArgument(AbstractInitializableBeanDefinition.java:1515)
	at io.micronaut.configuration.metrics.micrometer.$MeterRegistryFactory$CompositeMeterRegistry0$Definition.instantiate(Unknown Source)
	at io.micronaut.context.DefaultBeanContext.resolveByBeanFactory(DefaultBeanContext.java:2342)
	at io.micronaut.context.DefaultBeanContext.createRegistration(DefaultBeanContext.java:3162)
	at io.micronaut.context.SingletonScope.getOrCreate(SingletonScope.java:80)
	at io.micronaut.context.DefaultBeanContext.resolveBeanRegistration(DefaultBeanContext.java:3012)
	at io.micronaut.context.DefaultBeanContext.resolveBeanRegistration(DefaultBeanContext.java:2774)
	at io.micronaut.context.DefaultBeanContext.getBean(DefaultBeanContext.java:1786)
	at io.micronaut.context.AbstractBeanResolutionContext.getBean(AbstractBeanResolutionContext.java:210)
	at io.micronaut.context.AbstractInitializableBeanDefinition.resolveBean(AbstractInitializableBeanDefinition.java:2137)
	at io.micronaut.context.AbstractInitializableBeanDefinition.getBeanForField(AbstractInitializableBeanDefinition.java:1711)
	at com.example.$OtlpMeterRegistryDependencyDemoTest$Definition.inject(Unknown Source)
	at io.micronaut.context.DefaultBeanContext.doInjectAndInitialize(DefaultBeanContext.java:2679)
	at io.micronaut.context.DefaultBeanContext.inject(DefaultBeanContext.java:1014)
	at io.micronaut.test.extensions.AbstractMicronautExtension.beforeEach(AbstractMicronautExtension.java:476)
	at io.micronaut.test.extensions.junit5.MicronautJunit5Extension.beforeEach(MicronautJunit5Extension.java:236)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1604)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1604)
Caused by: java.lang.NoClassDefFoundError: com/google/protobuf/RuntimeVersion$RuntimeDomain
	at io.opentelemetry.proto.resource.v1.Resource.<clinit>(Resource.java:21)
	at io.micrometer.registry.otlp.OtlpMeterRegistry.<init>(OtlpMeterRegistry.java:130)
	at io.micrometer.registry.otlp.OtlpMeterRegistry.<init>(OtlpMeterRegistry.java:119)
	at io.micrometer.registry.otlp.OtlpMeterRegistry.<init>(OtlpMeterRegistry.java:108)
	at io.micronaut.configuration.metrics.micrometer.otlp.OtlpMeterRegistryFactory.otlpMeterRegistry(OtlpMeterRegistryFactory.java:51)
	at io.micronaut.configuration.metrics.micrometer.otlp.$OtlpMeterRegistryFactory$OtlpMeterRegistry0$Definition.instantiate(Unknown Source)
	at io.micronaut.context.DefaultBeanContext.resolveByBeanFactory(DefaultBeanContext.java:2342)
	... 27 more
Caused by: java.lang.ClassNotFoundException: com.google.protobuf.RuntimeVersion$RuntimeDomain
	at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:580)
	at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:490)
	... 34 more

The problem is that OtlpMeterRegistry ultimately depends on com.google.protobuf.RuntimeVersion, which exists in protobuf-java 4.x, but not 3.x. The micronaut-parent POM sets the protobuf version to 3.25.8.

This has been a problem since Micronaut 4.9.0, which bumped from a transitive dependency on io.opentelemetry.proto:opentelemetry-proto:1.2.0-alpha to ...:1.5.0-alpha, which is what introduces the protobuf-java 4.x dependency.

Steps To Reproduce

1. Create a new Micronaut project:

mn create-app --build=maven --jdk=21 --lang=java --test=junit com.example.otlp-meter-registry-dependency-demo

2. Add io.micronaut.micrometer:micronaut-micrometer-registry-otlp as a dependency and add @Inject MeterRegistry meterRegistry to the auto-generated test class:

diff --git a/pom.xml b/pom.xml
index 83be2e2..0080cd2 100644
--- a/pom.xml
+++ b/pom.xml
@@ -36,6 +36,10 @@
       <artifactId>micronaut-http-server-netty</artifactId>
       <scope>compile</scope>
     </dependency>
+    <dependency>
+      <groupId>io.micronaut.micrometer</groupId>
+      <artifactId>micronaut-micrometer-registry-otlp</artifactId>
+    </dependency>
     <dependency>
       <groupId>io.micronaut.serde</groupId>
       <artifactId>micronaut-serde-jackson</artifactId>
diff --git a/src/test/java/com/example/OtlpMeterRegistryDependencyDemoTest.java b/src/test/java/com/example/OtlpMeterRegistryDependencyDemoTest.java
index 9a4d6c2..78f3630 100644
--- a/src/test/java/com/example/OtlpMeterRegistryDependencyDemoTest.java
+++ b/src/test/java/com/example/OtlpMeterRegistryDependencyDemoTest.java
@@ -1,5 +1,6 @@
 package com.example;

+import io.micrometer.core.instrument.MeterRegistry;
 import io.micronaut.runtime.EmbeddedApplication;
 import io.micronaut.test.extensions.junit5.annotation.MicronautTest;
 import org.junit.jupiter.api.Test;
@@ -13,6 +14,9 @@ class OtlpMeterRegistryDependencyDemoTest {
     @Inject
     EmbeddedApplication<?> application;

+    @Inject
+    MeterRegistry meterRegistry;
+
     @Test
     void testItWorks() {
         Assertions.assertTrue(application.isRunning());

3. Run the test:

./mvnw clean test
...
[ERROR] Errors: 
[ERROR]   OtlpMeterRegistryDependencyDemoTest.testItWorks » BeanInstantiation Error instantiating bean of type  [io.micrometer.core.instrument.composite.CompositeMeterRegistry]

Message: com/google/protobuf/RuntimeVersion$RuntimeDomain
Path Taken:
c.e.OtlpMeterRegistryDependencyDemoTest#meterRegistry
\---> @i.m.c.a.Primary @j.i.Singleton i.m.c.i.c.CompositeMeterRegistry i.m.c.m.m.MeterRegistryFactory.compositeMeterRegistry#compositeMeterRegistry([List<MeterRegistry> registries], List<MeterRegistryConfigurer<MeterRegistry>> configurers)

Environment Information

Reproducible in all environments with Micronaut 4.9.0 or later.

Example Application

Please find an example application attached: otlp-meter-registry-dependency-demo.zip

…though the attached project is produced by creating a project with the CLI invocation given above and applying the patch included above.

Version

Micronaut 4.9.0 or later

[jon-signal]

In the spirit of clarity (rather than belaboring the point), I should state explicitly that I think the impact of this issue is that the OTLP registry simply does not work "out of the box" and has not since Micronaut 4.9.0.

The workaround we've found is to override the protobuf.version property from the parent POM, but it's not clear that doing so won't have other side effects (presumably if it were harmless and had no side effects, the parent POM would already be using 4.x for its protobuf.version). Then again: maybe it really is that simple!

[jon-signal]

In the spirit of clarity (rather than belaboring the point), I should state explicitly that I think the impact of this issue is that the OTLP registry simply does not work "out of the box" and has not since Micronaut 4.9.0.

Clarifying further: by "does not work," I mean that any application built with Micronaut 4.9 and micronaut-micrometer-registry-otlp will fail to start. This seems like a serious bug to me.