Avoid Servlet observations failures for invalid HTTP status

Prior to this commit, the `DefaultServerRequestObservationConvention`
for Servlet failed when the HTTP response status was invalid (for
example, set to "0").

This commit catches `IllegalArgumentException` thrown for such invalid
HTTP status and instead returns an unknown outcome for the observation.

Fixes gh-33725

spring-web/src/main/java/org/springframework/http/server/observation/DefaultServerRequestObservationConvention.java

@@ -160,9 +160,14 @@ public class DefaultServerRequestObservationConvention implements ServerRequestO
 	}
 
 	protected KeyValue outcome(ServerRequestObservationContext context) {
-		if (context.getResponse() != null) {
-			HttpStatusCode statusCode = HttpStatusCode.valueOf(context.getResponse().getStatus());
-			return HttpOutcome.forStatus(statusCode);
+		try {
+			if (context.getResponse() != null) {
+				HttpStatusCode statusCode = HttpStatusCode.valueOf(context.getResponse().getStatus());
+				return HttpOutcome.forStatus(statusCode);
+			}
+		}
+		catch (IllegalArgumentException ex) {
+			return HTTP_OUTCOME_UNKNOWN;
 		}
 		return HTTP_OUTCOME_UNKNOWN;
 	}

spring-web/src/test/java/org/springframework/http/server/observation/DefaultServerRequestObservationConventionTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2023 the original author or authors.
+ * Copyright 2002-2024 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.
@@ -137,4 +137,16 @@ class DefaultServerRequestObservationConventionTests {
 				.contains(KeyValue.of("http.url", "/test"));
 	}
 
+	@Test
+	void addsKeyValuesForInvalidStatusExchange() {
+		this.request.setRequestURI("/test/invalidStatus");
+		this.response.setStatus(0);
+
+		assertThat(this.convention.getLowCardinalityKeyValues(this.context)).hasSize(5)
+				.contains(KeyValue.of("method", "GET"), KeyValue.of("uri", "UNKNOWN"), KeyValue.of("status", "0"),
+						KeyValue.of("exception", "none"), KeyValue.of("outcome", "UNKNOWN"));
+		assertThat(this.convention.getHighCardinalityKeyValues(this.context)).hasSize(1)
+				.contains(KeyValue.of("http.url", "/test/invalidStatus"));
+	}
+
 }