Add support for Prometheus Exemplars

See gh-30472

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

@@ -25,11 +25,15 @@ import io.micrometer.core.instrument.Clock;
 import io.micrometer.prometheus.PrometheusConfig;
 import io.micrometer.prometheus.PrometheusMeterRegistry;
 import io.prometheus.client.CollectorRegistry;
+import io.prometheus.client.exemplars.DefaultExemplarSampler;
+import io.prometheus.client.exemplars.ExemplarSampler;
+import io.prometheus.client.exemplars.tracer.common.SpanContextSupplier;
 import io.prometheus.client.exporter.BasicAuthHttpConnectionFactory;
 import io.prometheus.client.exporter.PushGateway;
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
 
+import org.springframework.beans.factory.ObjectProvider;
 import org.springframework.boot.actuate.autoconfigure.endpoint.condition.ConditionalOnAvailableEndpoint;
 import org.springframework.boot.actuate.autoconfigure.metrics.CompositeMeterRegistryAutoConfiguration;
 import org.springframework.boot.actuate.autoconfigure.metrics.MetricsAutoConfiguration;
@@ -56,6 +60,7 @@ import org.springframework.util.StringUtils;
  *
  * @author Jon Schneider
  * @author David J. M. Karlsen
+ * @author Jonatan Ivanov
  * @since 2.0.0
  */
 @AutoConfiguration(
@@ -76,8 +81,9 @@ public class PrometheusMetricsExportAutoConfiguration {
 	@Bean
 	@ConditionalOnMissingBean
 	public PrometheusMeterRegistry prometheusMeterRegistry(PrometheusConfig prometheusConfig,
-			CollectorRegistry collectorRegistry, Clock clock) {
+			CollectorRegistry collectorRegistry, Clock clock, ObjectProvider<ExemplarSampler> exemplarSamplerProvider) {
-		return new PrometheusMeterRegistry(prometheusConfig, collectorRegistry, clock);
+		return new PrometheusMeterRegistry(prometheusConfig, collectorRegistry, clock,
+				exemplarSamplerProvider.getIfAvailable());
 	}
 
 	@Bean
@@ -86,6 +92,13 @@ public class PrometheusMetricsExportAutoConfiguration {
 		return new CollectorRegistry(true);
 	}
 
+	@Bean
+	@ConditionalOnMissingBean
+	@ConditionalOnBean(SpanContextSupplier.class)
+	public ExemplarSampler exemplarSampler(SpanContextSupplier spanContextSupplier) {
+		return new DefaultExemplarSampler(spanContextSupplier);
+	}
+
 	@Configuration(proxyBeanMethods = false)
 	@ConditionalOnAvailableEndpoint(endpoint = PrometheusScrapeEndpoint.class)
 	public static class PrometheusScrapeEndpointConfiguration {

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

@@ -22,6 +22,8 @@ import io.micrometer.core.instrument.Clock;
 import io.micrometer.prometheus.PrometheusConfig;
 import io.micrometer.prometheus.PrometheusMeterRegistry;
 import io.prometheus.client.CollectorRegistry;
+import io.prometheus.client.exemplars.ExemplarSampler;
+import io.prometheus.client.exemplars.tracer.common.SpanContextSupplier;
 import io.prometheus.client.exporter.BasicAuthHttpConnectionFactory;
 import io.prometheus.client.exporter.DefaultHttpConnectionFactory;
 import io.prometheus.client.exporter.HttpConnectionFactory;
@@ -50,6 +52,7 @@ import static org.assertj.core.api.Assertions.assertThat;
  *
  * @author Andy Wilkinson
  * @author Stephane Nicoll
+ * @author Jonatan Ivanov
  */
 @ExtendWith(OutputCaptureExtension.class)
 class PrometheusMetricsExportAutoConfigurationTests {
@@ -109,6 +112,20 @@ class PrometheusMetricsExportAutoConfigurationTests {
 						.hasSingleBean(PrometheusConfig.class));
 	}
 
+	@Test
+	void autoConfiguresExemplarSamplerIfSpanContextSupplierIsPresent() {
+		this.contextRunner.withUserConfiguration(ExemplarsConfiguration.class)
+				.run((context) -> assertThat(context).hasSingleBean(SpanContextSupplier.class)
+						.hasSingleBean(ExemplarSampler.class).hasSingleBean(PrometheusMeterRegistry.class));
+	}
+
+	@Test
+	void exemplarSamplerIsNotAutoConfiguredIfSpanContextSupplierIsMissing() {
+		this.contextRunner.withUserConfiguration(BaseConfiguration.class)
+				.run((context) -> assertThat(context).doesNotHaveBean(SpanContextSupplier.class)
+						.doesNotHaveBean(ExemplarSampler.class).hasSingleBean(PrometheusMeterRegistry.class));
+	}
+
 	@Test
 	void addsScrapeEndpointToManagementContext() {
 		this.contextRunner.withConfiguration(AutoConfigurations.of(ManagementContextAutoConfiguration.class))
@@ -271,4 +288,25 @@ class PrometheusMetricsExportAutoConfigurationTests {
 
 	}
 
+	@Configuration(proxyBeanMethods = false)
+	@Import(BaseConfiguration.class)
+	static class ExemplarsConfiguration {
+
+		@Bean
+		SpanContextSupplier spanContextSupplier() {
+			return new SpanContextSupplier() {
+				@Override
+				public String getTraceId() {
+					return null;
+				}
+
+				@Override
+				public String getSpanId() {
+					return null;
+				}
+			};
+		}
+
+	}
+
 }

spring-boot-project/spring-boot-docs/src/docs/asciidoc/actuator/metrics.adoc

@@ -455,6 +455,9 @@ The following example `scrape_config` adds to `prometheus.yml`:
 		  - targets: ["HOST:PORT"]
 ----
 
+https://prometheus.io/docs/prometheus/latest/feature_flags/#exemplars-storage[Prometheus Exemplars] are also supported. To enable this feature, a `SpanContextSupplier` bean should present. If you use https://spring.io/projects/spring-cloud-sleuth[Spring Cloud Sleuth], this will be auto-configured for you, but you can always create your own if you want. +
+Please check the https://prometheus.io/docs/prometheus/latest/feature_flags/#exemplars-storage[Prometheus Docs], since this feature needs to be explicitly enabled on Prometheus' side, and it is only supported using the https://github.com/OpenObservability/OpenMetrics/blob/v1.0.0/specification/OpenMetrics.md#exemplars[OpenMetrics] format.
+
 For ephemeral or batch jobs that may not exist long enough to be scraped, you can use https://github.com/prometheus/pushgateway[Prometheus Pushgateway] support to expose the metrics to Prometheus.
 To enable Prometheus Pushgateway support, add the following dependency to your project: