Register no-op tracer bean if tracing is disabled

Closes gh-32907

spring-boot-project/spring-boot-actuator-autoconfigure/src/main/java/org/springframework/boot/actuate/autoconfigure/tracing/TracingDisabledAutoConfiguration.java

@@ -0,0 +1,45 @@
+/*
+ * Copyright 2012-2022 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.actuate.autoconfigure.tracing;
+
+import io.micrometer.tracing.Tracer;
+
+import org.springframework.boot.autoconfigure.AutoConfiguration;
+import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
+import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
+import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
+import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
+import org.springframework.context.annotation.Bean;
+
+/**
+ * {@link EnableAutoConfiguration Auto-configuration} if tracing is disabled..
+ *
+ * @author Moritz Halbritter
+ * @since 3.0.0
+ */
+@AutoConfiguration(before = MicrometerTracingAutoConfiguration.class)
+@ConditionalOnClass(Tracer.class)
+@ConditionalOnProperty(prefix = "management.tracing", name = "enabled", havingValue = "false")
+public class TracingDisabledAutoConfiguration {
+
+	@Bean
+	@ConditionalOnMissingBean
+	Tracer noopTracer() {
+		return Tracer.NOOP;
+	}
+
+}

spring-boot-project/spring-boot-actuator-autoconfigure/src/main/resources/META-INF/spring/org.springframework.boot.autoconfigure.AutoConfiguration.imports

@@ -99,6 +99,7 @@ org.springframework.boot.actuate.autoconfigure.system.DiskSpaceHealthContributor
 org.springframework.boot.actuate.autoconfigure.tracing.BraveAutoConfiguration
 org.springframework.boot.actuate.autoconfigure.tracing.MicrometerTracingAutoConfiguration
 org.springframework.boot.actuate.autoconfigure.tracing.OpenTelemetryAutoConfiguration
+org.springframework.boot.actuate.autoconfigure.tracing.TracingDisabledAutoConfiguration
 org.springframework.boot.actuate.autoconfigure.tracing.exemplars.ExemplarsAutoConfiguration
 org.springframework.boot.actuate.autoconfigure.tracing.wavefront.WavefrontTracingAutoConfiguration
 org.springframework.boot.actuate.autoconfigure.tracing.zipkin.ZipkinAutoConfiguration

spring-boot-project/spring-boot-actuator-autoconfigure/src/test/java/org/springframework/boot/actuate/autoconfigure/tracing/TracingDisabledAutoConfigurationTests.java

@@ -0,0 +1,54 @@
+/*
+ * Copyright 2012-2022 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.actuate.autoconfigure.tracing;
+
+import io.micrometer.tracing.Tracer;
+import org.junit.jupiter.api.Test;
+
+import org.springframework.boot.autoconfigure.AutoConfigurations;
+import org.springframework.boot.test.context.runner.ApplicationContextRunner;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link TracingDisabledAutoConfiguration}.
+ *
+ * @author Moritz Halbritter
+ */
+class TracingDisabledAutoConfigurationTests {
+
+	private final ApplicationContextRunner contextRunner = new ApplicationContextRunner()
+			.withConfiguration(AutoConfigurations.of(TracingDisabledAutoConfiguration.class));
+
+	@Test
+	void shouldSupplyBeans() {
+		this.contextRunner.withPropertyValues("management.tracing.enabled=false")
+				.run((context) -> assertThat(context).hasSingleBean(Tracer.class));
+	}
+
+	@Test
+	void shouldNotSupplyBeansWhenTracingIsEnabled() {
+		this.contextRunner.withPropertyValues("management.tracing.enabled=true")
+				.run((context) -> assertThat(context).doesNotHaveBean(Tracer.class));
+	}
+
+	@Test
+	void shouldNotSupplyBeansWhenTracingIsImplicitelyEnabled() {
+		this.contextRunner.run((context) -> assertThat(context).doesNotHaveBean(Tracer.class));
+	}
+
+}