Caching number of primary shards per node for evaluating constraints on avg primary shards across all indices per node (opensearch-project#14992) (opensearch-project#15045)

(cherry picked from commit 4707885)

Signed-off-by: RS146BIJAY <rishavsagar4b1@gmail.com>
Signed-off-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

Author: opensearch-trigger-bot[bot]

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

@@ -476,6 +476,7 @@ void updateRebalanceConstraint(String constraint, boolean add) {
     public static class ModelNode implements Iterable<ModelIndex> {
         private final Map<String, ModelIndex> indices = new HashMap<>();
         private int numShards = 0;
+        private int numPrimaryShards = 0;
         private final RoutingNode routingNode;
 
         ModelNode(RoutingNode routingNode) {
@@ -509,7 +510,7 @@ public int numPrimaryShards(String idx) {
         }
 
         public int numPrimaryShards() {
-            return indices.values().stream().mapToInt(index -> index.numPrimaryShards()).sum();
+            return numPrimaryShards;
         }
 
         public int highestPrimary(String index) {
@@ -527,6 +528,10 @@ public void addShard(ShardRouting shard) {
                 indices.put(index.getIndexId(), index);
             }
             index.addShard(shard);
+            if (shard.primary()) {
+                numPrimaryShards++;
+            }
+
             numShards++;
         }
 
@@ -538,6 +543,11 @@ public void removeShard(ShardRouting shard) {
                     indices.remove(shard.getIndexName());
                 }
             }
+
+            if (shard.primary()) {
+                numPrimaryShards--;
+            }
+
             numShards--;
         }