Use String.join() to describe a list of tasks (elastic#28941)

This change replaces the use of string concatenation with a call to
String.join(). String concatenation might be quadratic, unless the compiler can
optimise it away, whereas String.join() is more reliably linear. There can
sometimes be a large number of pending ClusterState update tasks and elastic#28920
includes a report that this operation sometimes takes a long time.

core/src/main/java/org/elasticsearch/cluster/ClusterStateTaskExecutor.java

@@ -55,15 +55,7 @@ default void clusterStatePublished(ClusterChangedEvent clusterChangedEvent) {
      * This allows groupd task description but the submitting source.
      */
     default String describeTasks(List<T> tasks) {
-        return tasks.stream().map(T::toString).reduce((s1,s2) -> {
-            if (s1.isEmpty()) {
-                return s2;
-            } else if (s2.isEmpty()) {
-                return s1;
-            } else {
-                return s1 + ", " + s2;
-            }
-        }).orElse("");
+        return String.join(", ", tasks.stream().map(t -> (CharSequence)t.toString()).filter(t -> t.length() == 0)::iterator);
     }
 
     /**

core/src/main/java/org/elasticsearch/cluster/metadata/MetaDataMappingService.java

@@ -350,7 +350,7 @@ private ClusterState applyRequest(ClusterState currentState, PutMappingClusterSt
 
         @Override
         public String describeTasks(List<PutMappingClusterStateUpdateRequest> tasks) {
-            return tasks.stream().map(PutMappingClusterStateUpdateRequest::type).reduce((s1, s2) -> s1 + ", " + s2).orElse("");
+            return String.join(", ", tasks.stream().map(t -> (CharSequence)t.type())::iterator);
         }
     }