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
8c24e8c034