Add noop tracer bean if tracing is disabled

The new functionality doesn't expose any public API
and works the same as WebTestClientContextCustomizer
does.

Closes gh-32907

spring-boot-project/spring-boot-test-autoconfigure/src/main/java/org/springframework/boot/test/autoconfigure/actuate/observability/ObservabilityContextCustomizerFactory.java

@@ -19,17 +19,36 @@ package org.springframework.boot.test.autoconfigure.actuate.observability;
 import java.util.List;
 import java.util.Objects;
 
+import io.micrometer.tracing.Tracer;
+
+import org.springframework.aot.AotDetector;
+import org.springframework.beans.BeansException;
+import org.springframework.beans.factory.BeanFactory;
+import org.springframework.beans.factory.BeanFactoryAware;
+import org.springframework.beans.factory.BeanFactoryUtils;
+import org.springframework.beans.factory.ListableBeanFactory;
+import org.springframework.beans.factory.config.BeanDefinition;
+import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
+import org.springframework.beans.factory.support.BeanDefinitionRegistry;
+import org.springframework.beans.factory.support.BeanDefinitionRegistryPostProcessor;
+import org.springframework.beans.factory.support.RootBeanDefinition;
 import org.springframework.boot.test.util.TestPropertyValues;
 import org.springframework.context.ConfigurableApplicationContext;
+import org.springframework.context.annotation.ConfigurationClassPostProcessor;
+import org.springframework.core.Ordered;
 import org.springframework.test.context.ContextConfigurationAttributes;
 import org.springframework.test.context.ContextCustomizer;
 import org.springframework.test.context.ContextCustomizerFactory;
 import org.springframework.test.context.MergedContextConfiguration;
 import org.springframework.test.context.TestContextAnnotationUtils;
+import org.springframework.util.ClassUtils;
 
 /**
  * {@link ContextCustomizerFactory} that globally disables metrics export and tracing
  * unless {@link AutoConfigureObservability} is set on the test class.
+ * <p>
+ * Registers {@link Tracer#NOOP} if tracing is disabled, micrometer-tracing is on the
+ * classpath, and the user hasn't supplied their own {@link Tracer}.
  *
  * @author Chris Bono
  * @author Moritz Halbritter
@@ -44,11 +63,10 @@ class ObservabilityContextCustomizerFactory implements ContextCustomizerFactory
 		if (annotation == null) {
 			return new DisableObservabilityContextCustomizer(true, true);
 		}
-
 		return new DisableObservabilityContextCustomizer(!annotation.metrics(), !annotation.tracing());
 	}
 
-	static class DisableObservabilityContextCustomizer implements ContextCustomizer {
+	private static class DisableObservabilityContextCustomizer implements ContextCustomizer {
 
 		private final boolean disableMetrics;
 
@@ -68,9 +86,29 @@ class ObservabilityContextCustomizerFactory implements ContextCustomizerFactory
 			}
 			if (this.disableTracing) {
 				TestPropertyValues.of("management.tracing.enabled=false").applyTo(context);
+				registerNoopTracer(context);
 			}
 		}
 
+		private void registerNoopTracer(ConfigurableApplicationContext context) {
+			if (AotDetector.useGeneratedArtifacts()) {
+				return;
+			}
+			if (!ClassUtils.isPresent("io.micrometer.tracing.Tracer", context.getClassLoader())) {
+				return;
+			}
+			ConfigurableListableBeanFactory beanFactory = context.getBeanFactory();
+			if (beanFactory instanceof BeanDefinitionRegistry registry) {
+				registerNoopTracer(registry);
+			}
+		}
+
+		private void registerNoopTracer(BeanDefinitionRegistry registry) {
+			RootBeanDefinition definition = new RootBeanDefinition(NoopTracerRegistrar.class);
+			definition.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
+			registry.registerBeanDefinition(NoopTracerRegistrar.class.getName(), definition);
+		}
+
 		@Override
 		public boolean equals(Object o) {
 			if (this == o) {
@@ -90,4 +128,40 @@ class ObservabilityContextCustomizerFactory implements ContextCustomizerFactory
 
 	}
 
+	/**
+	 * {@link BeanDefinitionRegistryPostProcessor} that runs after the
+	 * {@link ConfigurationClassPostProcessor} and adds a {@link Tracer} bean definition
+	 * when a {@link Tracer} hasn't already been registered.
+	 */
+	private static class NoopTracerRegistrar implements BeanDefinitionRegistryPostProcessor, Ordered, BeanFactoryAware {
+
+		private BeanFactory beanFactory;
+
+		@Override
+		public void setBeanFactory(BeanFactory beanFactory) throws BeansException {
+			this.beanFactory = beanFactory;
+		}
+
+		@Override
+		public int getOrder() {
+			return Ordered.LOWEST_PRECEDENCE;
+		}
+
+		@Override
+		public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) throws BeansException {
+			if (AotDetector.useGeneratedArtifacts()) {
+				return;
+			}
+			if (BeanFactoryUtils.beanNamesForTypeIncludingAncestors((ListableBeanFactory) this.beanFactory,
+					Tracer.class, false, false).length == 0) {
+				registry.registerBeanDefinition("noopTracer", new RootBeanDefinition(Tracer.class, () -> Tracer.NOOP));
+			}
+		}
+
+		@Override
+		public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
+		}
+
+	}
+
 }