Verify sender propagation for HTTP client instrumentation

This will catch issues in the implementation of instrumentation. For example, if the carrier is set after an Observation is started, this new test will not pass.

micrometer-test/src/main/java/io/micrometer/core/instrument/HttpClientTimingInstrumentationVerificationTests.java

@@ -20,8 +20,10 @@
 import io.micrometer.common.lang.Nullable;
 import io.micrometer.core.annotation.Incubating;
 import io.micrometer.observation.Observation;
+import io.micrometer.observation.ObservationHandler;
 import io.micrometer.observation.ObservationRegistry;
-import org.junit.jupiter.api.Assumptions;
+import io.micrometer.observation.tck.TestObservationRegistry;
+import io.micrometer.observation.transport.RequestReplySenderContext;
 import org.junit.jupiter.api.Disabled;
 import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.EnumSource;
@@ -33,6 +35,7 @@
 
 import static com.github.tomakehurst.wiremock.client.WireMock.*;
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.junit.jupiter.api.Assumptions.assumeTrue;
 
 /**
  * Test suite for HTTP client timing instrumentation that verifies the expected metrics
@@ -212,9 +215,52 @@ void clientException(TestType testType, WireMockRuntimeInfo wmRuntimeInfo) {
         assertThat(timer.totalTime(TimeUnit.NANOSECONDS)).isPositive();
     }
 
+    // TODO this test doesn't need to be parameterized but the custom resolver for
+    // Before/After methods doesn't like when it isn't.
+    @ParameterizedTest
+    @EnumSource(TestType.class)
+    void headerIsPropagatedFromContext(TestType testType, WireMockRuntimeInfo wmRuntimeInfo) {
+        checkAndSetupTestForTestType(testType);
+
+        stubFor(get(anyUrl()).willReturn(ok()));
+
+        String templatedPath = "/fxrates/{currencypair}";
+        sendHttpRequest(instrumentedClient(testType), HttpMethod.GET, null, URI.create(wmRuntimeInfo.getHttpBaseUrl()),
+                templatedPath, "USDJPY");
+
+        // Only Observation-based instrumentation deals with propagation
+        // TODO documentation verification maybe shouldn't be done after each test?
+        // That's why this has to be after the http request is made
+        assumeTrue(testType == TestType.METRICS_VIA_OBSERVATIONS_WITH_METRICS_HANDLER);
+
+        verify(getRequestedFor(urlEqualTo("/fxrates/USDJPY")).withHeader("Test-Propagation", equalTo("testValue")));
+    }
+
+    @Override
+    protected TestObservationRegistry createObservationRegistryWithMetrics() {
+        TestObservationRegistry observationRegistryWithMetrics = super.createObservationRegistryWithMetrics();
+        observationRegistryWithMetrics.observationConfig().observationHandler(new SenderPropagationHandler<>());
+        return observationRegistryWithMetrics;
+    }
+
+    @SuppressWarnings({ "unchecked", "rawtypes" })
+    static class SenderPropagationHandler<T extends RequestReplySenderContext> implements ObservationHandler<T> {
+
+        @Override
+        public void onStart(T context) {
+            context.getSetter().set(context.getCarrier(), "Test-Propagation", "testValue");
+        }
+
+        @Override
+        public boolean supportsContext(Observation.Context context) {
+            return context instanceof RequestReplySenderContext;
+        }
+
+    }
+
     private void checkAndSetupTestForTestType(TestType testType) {
         if (testType == TestType.METRICS_VIA_OBSERVATIONS_WITH_METRICS_HANDLER) {
-            Assumptions.assumeTrue(clientInstrumentedWithObservations() != null,
+            assumeTrue(clientInstrumentedWithObservations() != null,
                     "You must implement the <clientInstrumentedWithObservations> method to test your instrumentation against an ObservationRegistry");
         }
     }