The following fields should be `final`, see also go/bugpattern/ImmutableEnumChecker https://github.com/googleapis/google-auth-library-java/blob/main/oauth2_http/java/com/google/auth/oauth2/IdTokenProvider.java#L62 https://github.com/googleapis/google-auth-library-java/blob/main/oauth2_http/java/com/google/auth/oauth2/MetricsUtils.java#L81