@Nullable
    public Instant notBefore() {
        return notBefore;
    }
{@value OAuth2Constants#NOT_BEFORE} Token Introspection Response field,
OPTIONAL. {@link Instant} timestamp, indicating when this token is not to be used before, as defined at
<a href="https://datatracker.ietf.org/doc/rfc7519/">[RFC7519]</a>.
oauth2/src/main/java/com/linecorp/armeria/common/auth/oauth2/OAuth2TokenDescriptor.java