Prometheus Exemplars Support

fixes gh-2039

spring-cloud-sleuth-instrumentation/pom.xml

@@ -249,6 +249,12 @@
 			<optional>true</optional>
 		</dependency>
 
+		<dependency>
+			<groupId>io.prometheus</groupId>
+			<artifactId>simpleclient_common</artifactId>
+			<optional>true</optional>
+		</dependency>
+
 		<dependency>
 			<groupId>org.springframework.boot</groupId>
 			<artifactId>spring-boot-starter-test</artifactId>

spring-cloud-sleuth-instrumentation/src/main/java/org/springframework/cloud/sleuth/instrument/prometheus/prometheus/SleuthSpanContextSupplier.java

@@ -0,0 +1,55 @@
+/*
+ * Copyright 2021-2021 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.cloud.sleuth.instrument.prometheus.prometheus;
+
+import io.prometheus.client.exemplars.tracer.common.SpanContextSupplier;
+
+import org.springframework.cloud.sleuth.Span;
+import org.springframework.cloud.sleuth.Tracer;
+
+/**
+ * {@link SpanContextSupplier} that integrates Sleuth's {@link Tracer} with Prometheus
+ * exemplars.
+ *
+ * @author Jonatan Ivanov
+ * @since 3.1.0
+ */
+public class SleuthSpanContextSupplier implements SpanContextSupplier {
+
+	private final Tracer tracer;
+
+	/**
+	 * @param tracer The tracer implementation to query for the current TraceId and
+	 * SpanId.
+	 */
+	public SleuthSpanContextSupplier(Tracer tracer) {
+		this.tracer = tracer;
+	}
+
+	@Override
+	public String getTraceId() {
+		Span span = tracer.currentSpan();
+		return span != null ? span.context().traceId() : null;
+	}
+
+	@Override
+	public String getSpanId() {
+		Span span = tracer.currentSpan();
+		return span != null ? span.context().spanId() : null;
+	}
+
+}

spring-cloud-sleuth-instrumentation/src/test/java/org/springframework/cloud/sleuth/instrument/prometheus/SleuthSpanContextSupplierTests.java

@@ -0,0 +1,69 @@
+/*
+ * Copyright 2021-2021 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.cloud.sleuth.instrument.prometheus;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.InjectMocks;
+import org.mockito.Mock;
+import org.mockito.junit.jupiter.MockitoExtension;
+
+import org.springframework.cloud.sleuth.Span;
+import org.springframework.cloud.sleuth.TraceContext;
+import org.springframework.cloud.sleuth.Tracer;
+import org.springframework.cloud.sleuth.instrument.prometheus.prometheus.SleuthSpanContextSupplier;
+
+import static org.assertj.core.api.BDDAssertions.assertThat;
+import static org.mockito.BDDMockito.mock;
+import static org.mockito.BDDMockito.when;
+
+/**
+ * Tests for {@link SleuthSpanContextSupplier}.
+ *
+ * @author Jonatan Ivanov
+ */
+@ExtendWith(MockitoExtension.class)
+class SleuthSpanContextSupplierTests {
+
+	@Mock
+	private Tracer tracer;
+
+	@InjectMocks
+	private SleuthSpanContextSupplier spanContextSupplier;
+
+	@Test
+	void should_provide_null_values_if_no_span_is_available() {
+		when(tracer.currentSpan()).thenReturn(null);
+
+		assertThat(spanContextSupplier.getTraceId()).isNull();
+		assertThat(spanContextSupplier.getSpanId()).isNull();
+	}
+
+	@Test
+	void should_provide_values_from_tracer_if_span_is_available() {
+		Span span = mock(Span.class);
+		TraceContext context = mock(TraceContext.class);
+		when(tracer.currentSpan()).thenReturn(span);
+		when(span.context()).thenReturn(context);
+		when(context.traceId()).thenReturn("42");
+		when(context.spanId()).thenReturn("24");
+
+		assertThat(spanContextSupplier.getTraceId()).isEqualTo("42");
+		assertThat(spanContextSupplier.getSpanId()).isEqualTo("24");
+	}
+
+}