public OAuth2TokenDescriptorBuilder userName(String userName) {
        this.userName = requireNonNull(userName, "userName");
        return this;
    }
{@code username} Token Introspection Response field,
OPTIONAL. Human-readable identifier for the resource owner who authorized this token.
oauth2/src/main/java/com/linecorp/armeria/common/auth/oauth2/OAuth2TokenDescriptorBuilder.java