public long remainingBackoffMs(long currentTimeMs) {
        if (!isBackingOff) {
            throw new IllegalStateException("Candidate is not currently backing off");
        }
        backoffTimer.update(currentTimeMs);
        return backoffTimer.remainingMs();
    }
Get the set of voters that have rejected our candidacy.

@return The set of rejecting voters
raft/src/main/java/org/apache/kafka/raft/CandidateState.java