public OAuth2TokenDescriptorBuilder notBefore(Instant notBefore) {
        this.notBefore = requireNonNull(notBefore, "notBefore");
        return this;
    }
{@code nbf} 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/OAuth2TokenDescriptorBuilder.java