Pre computing hash code for WeightedRouting to avoid recomputing it (#13099)

WeightedRoutingKey is used as key in the HashMap to retreive the shard
routings, for every usage the hash key is computed. WeightedRoutingKey
internally uses WeightedRouting. As the number of the shards getting
queried increases, the computation cost goes high.

Computing the hash key during object creation as the object is immutable
and using it through out the object thus saving on the additional computation.

Signed-off-by: Prabhakar Sithanandam <prabhakar.s87@gmail.com>

Author: backslasht

server/src/main/java/org/opensearch/cluster/routing/WeightedRouting.java

@@ -14,6 +14,7 @@
 import org.opensearch.core.common.io.stream.Writeable;
 
 import java.io.IOException;
+import java.util.Collections;
 import java.util.HashMap;
 import java.util.Map;
 import java.util.Objects;
@@ -25,27 +26,26 @@
  */
 @PublicApi(since = "2.4.0")
 public class WeightedRouting implements Writeable {
-    private String attributeName;
-    private Map<String, Double> weights;
+    private final String attributeName;
+    private final Map<String, Double> weights;
+    private final int hashCode;
 
     public WeightedRouting() {
-        this.attributeName = "";
-        this.weights = new HashMap<>(3);
+        this("", new HashMap<>(3));
     }
 
     public WeightedRouting(String attributeName, Map<String, Double> weights) {
         this.attributeName = attributeName;
-        this.weights = weights;
+        this.weights = Collections.unmodifiableMap(weights);
+        this.hashCode = Objects.hash(this.attributeName, this.weights);
     }
 
     public WeightedRouting(WeightedRouting weightedRouting) {
-        this.attributeName = weightedRouting.attributeName();
-        this.weights = weightedRouting.weights;
+        this(weightedRouting.attributeName(), weightedRouting.weights);
     }
 
     public WeightedRouting(StreamInput in) throws IOException {
-        attributeName = in.readString();
-        weights = (Map<String, Double>) in.readGenericValue();
+        this(in.readString(), (Map<String, Double>) in.readGenericValue());
     }
 
     public boolean isSet() {
@@ -70,7 +70,7 @@ public boolean equals(Object o) {
 
     @Override
     public int hashCode() {
-        return Objects.hash(attributeName, weights);
+        return hashCode;
     }
 
     @Override