Skip rebalancing when cluster_concurrent_rebalance threshold reached (#…

…33329)

Allows to skip shard balancing when the cluster_concurrent_rebalance threshold is already reached, which cuts down the time spent in the rebalance method of BalancedShardsAllocator.

server/src/main/java/org/elasticsearch/cluster/routing/allocation/decider/ConcurrentRebalanceAllocationDecider.java

@@ -61,6 +61,11 @@ private void setClusterConcurrentRebalance(int concurrentRebalance) {
 
     @Override
     public Decision canRebalance(ShardRouting shardRouting, RoutingAllocation allocation) {
+        return canRebalance(allocation);
+    }
+
+    @Override
+    public Decision canRebalance(RoutingAllocation allocation) {
         if (clusterConcurrentRebalance == -1) {
             return allocation.decision(Decision.YES, NAME, "unlimited concurrent rebalances are allowed");
         }