@Nullable
    public Duration expiresIn() {
        if (issuedAt != null && expiresAt != null) {
            return Duration.ofMillis(issuedAt.until(expiresAt, ChronoUnit.MILLIS));
        }
        return null;
    }
{@link Duration} indicating the lifetime of the access token. The value is NOT supplied with the
Token Introspection response and calculated based on {@value OAuth2Constants#ISSUED_AT} and
{@value OAuth2Constants#EXPIRES_AT} response fields every time this method invoked.
oauth2/src/main/java/com/linecorp/armeria/common/auth/oauth2/OAuth2TokenDescriptor.java