Merge branch '3.1.x'

Closes gh-36430

Author: snicoll

spring-boot-project/spring-boot-actuator-autoconfigure/src/main/java/org/springframework/boot/actuate/autoconfigure/metrics/export/prometheus/PrometheusMetricsExportAutoConfiguration.java

@@ -90,7 +90,7 @@ public CollectorRegistry collectorRegistry() {
 	}
 
 	@Bean
-	@ConditionalOnMissingBean
+	@ConditionalOnMissingBean(ExemplarSampler.class)
 	@ConditionalOnBean(SpanContextSupplier.class)
 	public DefaultExemplarSampler exemplarSampler(SpanContextSupplier spanContextSupplier) {
 		return new DefaultExemplarSampler(spanContextSupplier);

spring-boot-project/spring-boot-actuator-autoconfigure/src/test/java/org/springframework/boot/actuate/autoconfigure/metrics/export/prometheus/PrometheusMetricsExportAutoConfigurationTests.java

@@ -45,6 +45,7 @@
 import org.springframework.test.util.ReflectionTestUtils;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.Mockito.mock;
 
 /**
  * Tests for {@link PrometheusMetricsExportAutoConfiguration}.
@@ -125,6 +126,15 @@ void autoConfiguresExemplarSamplerIfSpanContextSupplierIsPresent() {
 				.hasSingleBean(PrometheusMeterRegistry.class));
 	}
 
+	@Test
+	void allowsCustomExemplarSamplerToBeUsed() {
+		this.contextRunner.withUserConfiguration(ExemplarsConfiguration.class)
+			.withBean("customExemplarSampler", ExemplarSampler.class, () -> mock(ExemplarSampler.class))
+			.run((context) -> assertThat(context).hasSingleBean(ExemplarSampler.class)
+				.getBean(ExemplarSampler.class)
+				.isSameAs(context.getBean("customExemplarSampler")));
+	}
+
 	@Test
 	void exemplarSamplerIsNotAutoConfiguredIfSpanContextSupplierIsMissing() {
 		this.contextRunner.withUserConfiguration(BaseConfiguration.class)