@Nullable
    public Instant expiresAt() {
        if (expiresIn == null) {
            return null;
        }
        if (expiresAt == null) {
            expiresAt = issuedAt.plus(expiresIn);
        }
        return expiresAt;
    }
An {@link Instant} representing a derived value using {@code issuedAt() + expiresIn()}.
oauth2/src/main/java/com/linecorp/armeria/common/auth/oauth2/GrantedOAuth2AccessToken.java