Add micrometer tracing auto-configuration tests

See spring-projectsgh-30156

Author: mhalbritter

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

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+/*
+ * 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 io.micrometer.tracing.handler.DefaultTracingObservationHandler;
+import org.junit.jupiter.api.Test;
+import org.mockito.Mockito;
+
+import org.springframework.boot.autoconfigure.AutoConfigurations;
+import org.springframework.boot.test.context.FilteredClassLoader;
+import org.springframework.boot.test.context.runner.ApplicationContextRunner;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link MicrometerTracingAutoConfiguration}.
+ *
+ * @author Moritz Halbritter
+ */
+class MicrometerTracingAutoConfigurationTests {
+
+	private final ApplicationContextRunner contextRunner = new ApplicationContextRunner()
+			.withConfiguration(AutoConfigurations.of(MicrometerTracingAutoConfiguration.class));
+
+	@Test
+	void shouldNotFailIfMicrometerIsMissing() {
+		this.contextRunner.withClassLoader(new FilteredClassLoader("io.micrometer"))
+				.run((context) -> assertThat(context).doesNotHaveBean(DefaultTracingObservationHandler.class));
+	}
+
+	@Test
+	void shouldHaveDefaultTracingObservationHandler() {
+		this.contextRunner.withUserConfiguration(TracerConfiguration.class)
+				.run((context) -> assertThat(context).hasSingleBean(DefaultTracingObservationHandler.class));
+	}
+
+	@Configuration(proxyBeanMethods = false)
+	static class TracerConfiguration {
+
+		@Bean
+		Tracer tracer() {
+			return Mockito.mock(Tracer.class);
+		}
+
+	}
+
+}