public OAuth2AuthorizationGrantBuilder refreshBefore(Duration refreshBefore) {
        requireNonNull(refreshBefore, "refreshBefore");
        checkState(!refreshBefore.isNegative(), "refreshBefore: %s (expected: >= 0)", refreshBefore);
        this.refreshBefore = refreshBefore;
        return this;
    }
Sets a period when the token should be refreshed proactively prior to its expiry.
oauth2/src/main/java/com/linecorp/armeria/client/auth/oauth2/OAuth2AuthorizationGrantBuilder.java