private boolean isVoter(ReplicaKey remoteReplicaKey) {
        ReplicaState state = voterStates.get(remoteReplicaKey.id());
        return state != null && state.matchesKey(remoteReplicaKey);
    }
Clear observer states that have not been active for a while and are not the leader.
raft/src/main/java/org/apache/kafka/raft/LeaderState.java