KAFKA-18131: Improve logs for voters (#18028)

Currently, the log of LeaderState#timeUntilCheckQuorumExpires uses streams without a terminal operator, resulting in output like java.util.stream.ReferencePipeline$3@39660237.
This PR aims to fix this issue and improve the log message.

Reviewers: Luke Chen <showuon@gmail.com>

raft/src/main/java/org/apache/kafka/raft/LeaderState.java

@@ -156,7 +156,10 @@ public long timeUntilCheckQuorumExpires(long currentTimeMs) {
                 "Current fetched voters are {}, and voters are {}",
                 checkQuorumTimeoutMs,
                 fetchedVoters,
-                voterStates.values().stream().map(voter -> voter.replicaKey)
+                voterStates.values()
+                    .stream()
+                    .map(voter -> voter.replicaKey)
+                    .collect(Collectors.toUnmodifiableSet())
             );
         }
         return remainingMs;

raft/src/main/java/org/apache/kafka/raft/ReplicaKey.java

@@ -70,7 +70,7 @@ public int hashCode() {
 
     @Override
     public String toString() {
-        return String.format("ReplicaKey(id=%d, directoryId=%s)", id, directoryId);
+        return String.format("ReplicaKey(id=%d, directoryId=%s)", id, directoryId.map(Uuid::toString).orElse("<undefined>"));
     }
 
     public static ReplicaKey of(int id, Uuid directoryId) {