Repository: quarkusio/quarkus
Issue: IllegalAccessError for gauges with Micrometer + MP Metrics binder

### Describe the bug

Creating an annotated `org.eclipse.microprofile.metrics.annotation.Gauge` when I use the Micrometer with the MP Metrics binder causes an error during gauge registration:

```
2021-09-10 07:40:04,106 ERROR [io.qua.run.Application] (Quarkus Main Thread) Failed to start application (with profile dev): java.lang.IllegalAccessError: failed to access class io.quarkus.micrometer.runtime.binder.mpmetrics.MpMetadata from class org.acme.GreetingResource_GaugeAdapter_Subclass (io.quarkus.micrometer.runtime.binder.mpmetrics.MpMetadata is in unnamed module of loader io.quarkus.bootstrap.classloading.QuarkusClassLoader @a73ea75; org.acme.GreetingResource_GaugeAdapter_Subclass is in unnamed module of loader io.quarkus.bootstrap.classloading.QuarkusClassLoader @4fdd30b2)
	at org.acme.GreetingResource_GaugeAdapter_Subclass.getMetadata(GreetingResource_GaugeAdapter_Subclass.zig:732)
	at org.acme.GreetingResource_GaugeAdapter_ClientProxy.getMetadata(GreetingResource_GaugeAdapter_ClientProxy.zig:276)
	at io.quarkus.micrometer.runtime.binder.mpmetrics.MetricRegistryAdapter.bindAnnotatedGauge(MetricRegistryAdapter.java:233)
	at io.quarkus.micrometer.runtime.binder.mpmetrics.MpMetricsBinder.bindTo(MpMetricsBinder.java:30)
	at io.quarkus.micrometer.runtime.binder.mpmetrics.MpMetricsBinder_Subclass.bindTo$$superforward1(MpMetricsBinder_Subclass.zig:98)
	at io.quarkus.micrometer.runtime.binder.mpmetrics.MpMetricsBinder_Subclass$$function$$1.apply(MpMetricsBinder_Subclass$$function$$1.zig:33)
	at io.quarkus.arc.impl.AroundInvokeInvocationContext.proceed(AroundInvokeInvocationContext.java:54)
	at io.quarkus.arc.runtime.devconsole.InvocationInterceptor.proceed(InvocationInterceptor.java:62)
	at io.quarkus.arc.runtime.devconsole.InvocationInterceptor.monitor(InvocationInterceptor.java:51)
	at io.quarkus.arc.runtime.devconsole.InvocationInterceptor_Bean.intercept(InvocationInterceptor_Bean.zig:521)
	at io.quarkus.arc.impl.InterceptorInvocation.invoke(InterceptorInvocation.java:41)
	at io.quarkus.arc.impl.AroundInvokeInvocationContext.perform(AroundInvokeInvocationContext.java:41)
	at io.quarkus.arc.impl.InvocationContexts.performAroundInvoke(InvocationContexts.java:32)
	at io.quarkus.micrometer.runtime.binder.mpmetrics.MpMetricsBinder_Subclass.bindTo(MpMetricsBinder_Subclass.zig:162)
	at io.quarkus.micrometer.runtime.MicrometerRecorder.lambda$configureRegistries$4(MicrometerRecorder.java:130)
	at java.base/java.lang.Iterable.forEach(Iterable.java:75)
	at io.quarkus.micrometer.runtime.MicrometerRecorder.configureRegistries(MicrometerRecorder.java:130)
	at io.quarkus.deployment.steps.MicrometerProcessor$configureRegistry-2125003310.deploy_0(MicrometerProcessor$configureRegistry-2125003310.zig:127)
	at io.quarkus.deployment.steps.MicrometerProcessor$configureRegistry-2125003310.deploy(MicrometerProcessor$configureRegistry-2125003310.zig:40)
	at io.quarkus.runner.ApplicationImpl.doStart(ApplicationImpl.zig:766)
	at io.quarkus.runtime.Application.start(Application.java:101)
	at io.quarkus.runtime.ApplicationLifecycleManager.run(ApplicationLifecycleManager.java:101)
	at io.quarkus.runtime.Quarkus.run(Quarkus.java:66)
	at io.quarkus.runtime.Quarkus.run(Quarkus.java:42)
	at io.quarkus.runtime.Quarkus.run(Quarkus.java:119)
	at io.quarkus.runner.GeneratedMain.main(GeneratedMain.zig:29)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:566)
	at io.quarkus.runner.bootstrap.StartupActionImpl$1.run(StartupActionImpl.java:100)
	at java.base/java.lang.Thread.run(Thread.java:834)

```

### Expected behavior

_No response_

### Actual behavior

_No response_

### How to Reproduce?

_No response_

### Output of `uname -a` or `ver`

_No response_

### Output of `java -version`

openjdk version "11.0.10" 2021-01-19 OpenJDK Runtime Environment GraalVM CE 21.0.0.2 (build 11.0.10+8-jvmci-21.0-b06) OpenJDK 64-Bit Server VM GraalVM CE 21.0.0.2 (build 11.0.10+8-jvmci-21.0-b06, mixed mode, sharing)

### GraalVM version (if different from Java)

_No response_

### Quarkus version or git rev

Quarkus 2.2.2

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

_No response_

### Additional information

Happens only in JVM mode

Changed files:
extensions/micrometer/runtime/src/main/java/io/quarkus/micrometer/runtime/binder/mpmetrics/MpMetadata.java

diff --git a/extensions/micrometer/runtime/src/main/java/io/quarkus/micrometer/runtime/binder/mpmetrics/MpMetadata.java b/extensions/micrometer/runtime/src/main/java/io/quarkus/micrometer/runtime/binder/mpmetrics/MpMetadata.java
index 66e95bdd4d8..f0962d19615 100644
--- a/extensions/micrometer/runtime/src/main/java/io/quarkus/micrometer/runtime/binder/mpmetrics/MpMetadata.java
+++ b/extensions/micrometer/runtime/src/main/java/io/quarkus/micrometer/runtime/binder/mpmetrics/MpMetadata.java
@@ -7,7 +7,7 @@
 import org.eclipse.microprofile.metrics.MetricType;
 import org.eclipse.microprofile.metrics.annotation.Metric;
 
-class MpMetadata implements Metadata {
+public class MpMetadata implements Metadata {
 
     public static MpMetadata sanitize(Metadata metadata, MetricType type) {
         if (metadata instanceof MpMetadata) {