Fallback on underlying server default when determining outcome tag

This commit also changed the default outcome to SUCCESS

Fixes gh-19367

spring-boot-project/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/web/reactive/server/WebFluxTags.java

@@ -136,14 +136,17 @@ public final class WebFluxTags {
 	 */
 	public static Tag outcome(ServerWebExchange exchange) {
 		Integer statusCode = extractStatusCode(exchange);
-		Outcome outcome = (statusCode != null) ? Outcome.forStatus(statusCode) : Outcome.UNKNOWN;
+		Outcome outcome = (statusCode != null) ? Outcome.forStatus(statusCode) : Outcome.SUCCESS;
 		return outcome.asTag();
 	}
 
 	private static Integer extractStatusCode(ServerWebExchange exchange) {
 		ServerHttpResponse response = exchange.getResponse();
 		if (response instanceof AbstractServerHttpResponse) {
-			return ((AbstractServerHttpResponse) response).getStatusCodeValue();
+			Integer statusCode = ((AbstractServerHttpResponse) response).getStatusCodeValue();
+			if (statusCode != null) {
+				return statusCode;
+			}
 		}
 		HttpStatus status = response.getStatusCode();
 		return (status != null) ? status.value() : null;

spring-boot-project/spring-boot-actuator/src/test/java/org/springframework/boot/actuate/metrics/web/reactive/server/WebFluxTagsTests.java

@@ -21,6 +21,7 @@ import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 
 import org.springframework.http.HttpStatus;
+import org.springframework.http.server.reactive.AbstractServerHttpResponse;
 import org.springframework.http.server.reactive.ServerHttpRequest;
 import org.springframework.mock.http.server.reactive.MockServerHttpRequest;
 import org.springframework.mock.web.server.MockServerWebExchange;
@@ -37,6 +38,7 @@ import static org.mockito.Mockito.mock;
  *
  * @author Brian Clozel
  * @author Michael McFadyen
+ * @author Madhura Bhave
  */
 class WebFluxTagsTests {
 
@@ -114,7 +116,20 @@ class WebFluxTagsTests {
 	void outcomeTagIsUnknownWhenResponseStatusIsNull() {
 		this.exchange.getResponse().setStatusCode(null);
 		Tag tag = WebFluxTags.outcome(this.exchange);
-		assertThat(tag.getValue()).isEqualTo("UNKNOWN");
+		assertThat(tag.getValue()).isEqualTo("SUCCESS");
+	}
+
+	@Test
+	void outcomeTagIsSuccessWhenResponseStatusIsAvailableFromUnderlyingServer() {
+		ServerWebExchange exchange = mock(ServerWebExchange.class);
+		ServerHttpRequest request = mock(ServerHttpRequest.class);
+		AbstractServerHttpResponse response = mock(AbstractServerHttpResponse.class);
+		given(response.getStatusCode()).willReturn(HttpStatus.OK);
+		given(response.getStatusCodeValue()).willReturn(null);
+		given(exchange.getRequest()).willReturn(request);
+		given(exchange.getResponse()).willReturn(response);
+		Tag tag = WebFluxTags.outcome(exchange);
+		assertThat(tag.getValue()).isEqualTo("SUCCESS");
 	}
 
 	@Test