Back off if spring-boot-opentelemetry is missing

Closes gh-46303

Author: mhalbritter

module/spring-boot-metrics/src/main/java/org/springframework/boot/metrics/autoconfigure/export/otlp/OtlpMetricsExportAutoConfiguration.java

@@ -51,7 +51,7 @@
 		before = { CompositeMeterRegistryAutoConfiguration.class, SimpleMetricsExportAutoConfiguration.class },
 		after = MetricsAutoConfiguration.class)
 @ConditionalOnBean(Clock.class)
-@ConditionalOnClass(OtlpMeterRegistry.class)
+@ConditionalOnClass({ OtlpMeterRegistry.class, OpenTelemetryProperties.class })
 @ConditionalOnEnabledMetricsExport("otlp")
 @EnableConfigurationProperties({ OtlpMetricsProperties.class, OpenTelemetryProperties.class })
 public final class OtlpMetricsExportAutoConfiguration {

module/spring-boot-metrics/src/test/java/org/springframework/boot/metrics/autoconfigure/export/otlp/OtlpMetricsExportAutoConfigurationTests.java

@@ -28,6 +28,7 @@
 
 import org.springframework.boot.autoconfigure.AutoConfigurations;
 import org.springframework.boot.metrics.autoconfigure.export.otlp.OtlpMetricsExportAutoConfiguration.PropertiesOtlpMetricsConnectionDetails;
+import org.springframework.boot.test.context.FilteredClassLoader;
 import org.springframework.boot.test.context.assertj.AssertableApplicationContext;
 import org.springframework.boot.test.context.runner.ApplicationContextRunner;
 import org.springframework.boot.testsupport.assertj.ScheduledExecutorServiceAssert;
@@ -41,6 +42,7 @@
  * Tests for {@link OtlpMetricsExportAutoConfiguration}.
  *
  * @author Eddú Meléndez
+ * @author Moritz Halbritter
  */
 class OtlpMetricsExportAutoConfigurationTests {
 
@@ -148,6 +150,13 @@ void allowsCustomMetricsSenderToBeUsedWithVirtualThreads() {
 			.run(this::assertHasCustomMetricsSender);
 	}
 
+	@Test
+	void shouldBackOffIfSpringBootOpenTelemetryIsMissing() {
+		this.contextRunner.withUserConfiguration(BaseConfiguration.class)
+			.withClassLoader(new FilteredClassLoader("org.springframework.boot.opentelemetry"))
+			.run((context) -> assertThat(context).doesNotHaveBean(OtlpMetricsExportAutoConfiguration.class));
+	}
+
 	private void assertHasCustomMetricsSender(AssertableApplicationContext context) {
 		assertThat(context).hasSingleBean(OtlpMeterRegistry.class);
 		OtlpMeterRegistry registry = context.getBean(OtlpMeterRegistry.class);