Properly include Prometheus related code

The way the test was being done previously
was checking the build time classpath instead
of the runtime class for the presence of the
Prometheus Registry.

Fixes: #33547
(cherry picked from commit a42d2de)

extensions/micrometer/deployment/src/main/java/io/quarkus/micrometer/deployment/export/PrometheusRegistryProcessor.java

@@ -39,7 +39,8 @@ public static class PrometheusEnabled implements BooleanSupplier {
         MicrometerConfig mConfig;
 
         public boolean getAsBoolean() {
-            return REGISTRY_CLASS != null && mConfig.checkRegistryEnabledWithDefault(mConfig.export.prometheus);
+            return (REGISTRY_CLASS != null) && QuarkusClassLoader.isClassPresentAtRuntime(REGISTRY_CLASS_NAME)
+                    && mConfig.checkRegistryEnabledWithDefault(mConfig.export.prometheus);
         }
     }