Add HttpOutcome for HTTP observations

Prior to this commit, the HTTP Observations would use
`HttpStatus.Series` as a value source for the "outcome" key value in
recorded observations. This would work for most cases, but would not
align in the 2xx HTTP status cases: the series would provide a
"SUCESSFUL" value whereas the heritage metrics support in Spring Boot
would give "SUCESS".

This commit introduces a dedicated `HttpOutcome` concept for this and
applies it to all HTTP observations.

Fixes gh-29232

spring-webflux/src/test/java/org/springframework/web/reactive/function/client/DefaultClientObservationConventionTests.java

@@ -83,7 +83,7 @@ class DefaultClientObservationConventionTests {
 		context.setUriTemplate("/resource/{id}");
 		assertThat(this.observationConvention.getLowCardinalityKeyValues(context))
 				.contains(KeyValue.of("exception", "none"), KeyValue.of("method", "GET"), KeyValue.of("uri", "/resource/{id}"),
-						KeyValue.of("status", "200"), KeyValue.of("outcome", "SUCCESSFUL"));
+						KeyValue.of("status", "200"), KeyValue.of("outcome", "SUCCESS"));
 		assertThat(this.observationConvention.getHighCardinalityKeyValues(context)).hasSize(2)
 				.contains(KeyValue.of("client.name", "none"), KeyValue.of("uri.expanded", "/resource/42"));
 	}

spring-webflux/src/test/java/org/springframework/web/reactive/function/client/DefaultClientObservationTests.java

@@ -66,7 +66,7 @@ public class DefaultClientObservationTests {
 				.retrieve().bodyToMono(Void.class).block(Duration.ofSeconds(10));
 		verifyAndGetRequest();
 
-		assertThatHttpObservation().hasLowCardinalityKeyValue("outcome", "SUCCESSFUL")
+		assertThatHttpObservation().hasLowCardinalityKeyValue("outcome", "SUCCESS")
 				.hasLowCardinalityKeyValue("uri", "/resource/{id}");
 	}