Speed up BalancedShardAllocator a Little (#84641)

`addShard` is quite hot when profiling in many-shards benchmarks.
This saves one lookup in the hot loop.
Also fixed the confusing naming in `ModelIndex`.

server/src/main/java/org/elasticsearch/cluster/routing/allocation/allocator/BalancedShardsAllocator.java

@@ -1102,8 +1102,8 @@ public class BalancedShardsAllocator implements ShardsAllocator {
             this.routingNode = routingNode;
         }
 
-        public ModelIndex getIndex(String indexId) {
-            return indices.get(indexId);
+        public ModelIndex getIndex(String indexName) {
+            return indices.get(indexName);
         }
 
         public String getNodeId() {
@@ -1132,12 +1132,7 @@ public class BalancedShardsAllocator implements ShardsAllocator {
         }
 
         public void addShard(ShardRouting shard) {
-            ModelIndex index = indices.get(shard.getIndexName());
-            if (index == null) {
-                index = new ModelIndex(shard.getIndexName());
-                indices.put(index.getIndexId(), index);
-            }
-            index.addShard(shard);
+            indices.computeIfAbsent(shard.getIndexName(), t -> new ModelIndex()).addShard(shard);
             numShards++;
         }
 
@@ -1172,13 +1167,10 @@ public class BalancedShardsAllocator implements ShardsAllocator {
     }
 
     static final class ModelIndex implements Iterable<ShardRouting> {
-        private final String id;
         private final Set<ShardRouting> shards = new HashSet<>(4); // expect few shards of same index to be allocated on same node
         private int highestPrimary = -1;
 
-        ModelIndex(String id) {
-            this.id = id;
-        }
+        ModelIndex() {}
 
         public int highestPrimary() {
             if (highestPrimary == -1) {
@@ -1193,10 +1185,6 @@ public class BalancedShardsAllocator implements ShardsAllocator {
             return highestPrimary;
         }
 
-        public String getIndexId() {
-            return id;
-        }
-
         public int numShards() {
             return shards.size();
         }