private long pollUnattachedAsObserver(UnattachedState state, long currentTimeMs) {
        long fetchBackoffMs = maybeSendAnyVoterFetch(currentTimeMs);
        return Math.min(fetchBackoffMs, state.remainingElectionTimeMs(currentTimeMs));
    }
Attempt to send a request. Return the time to wait before the request can be retried.
raft/src/main/java/org/apache/kafka/raft/KafkaRaftClient.java