MINOR: Consumer tidying (apache#21832)

Resolves a couple of review comments from
apache#21711. The remaining comments are
in `ShareConsumerTest` and will be addressed separately.

Reviewers: Lianet Magrans <lmagrans@confluent.io>, Viktor Somogyi-Vass
 <viktorsomogyi@gmail.com>, Chia-Ping Tsai <chia7712@gmail.com>

Author: AndrewJSchofield

clients/src/main/java/org/apache/kafka/clients/consumer/internals/AsyncKafkaConsumer.java

@@ -1824,15 +1824,15 @@ public Set<TopicPartition> assignment() {
     }
 
     /**
-     * Get the current subscription.  or an empty set if no such call has
+     * Get the current subscription, or an empty set if no such call has
      * been made.
      * @return The set of topics currently subscribed to
      */
     @Override
     public Set<String> subscription() {
         acquireAndEnsureOpen();
         try {
-            return Collections.unmodifiableSet(subscriptions.subscription());
+            return Set.copyOf(subscriptions.subscription());
         } finally {
             release();
         }

clients/src/main/java/org/apache/kafka/clients/consumer/internals/ShareConsumerImpl.java

@@ -81,6 +81,7 @@
 import java.time.Duration;
 import java.util.Arrays;
 import java.util.Collection;
+import java.util.Collections;
 import java.util.ConcurrentModificationException;
 import java.util.HashMap;
 import java.util.LinkedList;
@@ -958,7 +959,7 @@ public Optional<Integer> acquisitionLockTimeoutMs() {
      */
     @Override
     public Map<MetricName, ? extends Metric> metrics() {
-        return Map.copyOf(metrics.metrics());
+        return Collections.unmodifiableMap(metrics.metrics());
     }
 
     /**