public Instant issuedAt() {
        return issuedAt;
    }
An {@link Instant} indicating when the Access Token was issued.
The value is NOT supplied with the Access Token response and calculated approximately using
{@value OAuth2Constants#EXPIRES_IN} field.
oauth2/src/main/java/com/linecorp/armeria/common/auth/oauth2/GrantedOAuth2AccessToken.java