chore: add approx heap size methods to index classes

Author: eduardoramirez

hollow/src/main/java/com/netflix/hollow/core/index/HollowHashIndex.java

@@ -296,6 +296,15 @@ public String[] getMatchFields() {
         return matchFields;
     }
 
+    public long approxHeapFootprintInBytes() {
+        HollowHashIndexState hashState = hashStateVolatile;
+        if (hashState == null) {
+            return 0;
+        }
+        return hashState.getMatchHashTable().approxHeapFootprintInBytes() +
+          hashState.getSelectHashArray().approxHeapFootprintInBytes();
+    }
+
     protected static class HollowHashIndexState {
 
         final FixedLengthElementArray selectHashArray;

hollow/src/main/java/com/netflix/hollow/core/index/HollowPrimaryKeyIndex.java

@@ -656,6 +656,14 @@ public Object[] getRecordKey(int ordinal) {
         return keyDeriver.getRecordKey(ordinal);
     }
 
+    public long approxHeapFootprintInBytes() {
+        PrimaryKeyIndexHashTable state = hashTableVolatile;
+        if (state == null) {
+            return 0;
+        }
+        return state.hashTable.approxHeapFootprintInBytes();
+    }
+
     private boolean recordsHaveEqualKeys(int ordinal1, int ordinal2) {
         for(int i=0;i<fieldPathIndexes.length;i++) {
             if(!fieldsAreEqual(ordinal1, ordinal2, i))

hollow/src/main/java/com/netflix/hollow/core/index/HollowUniqueKeyIndex.java

@@ -697,6 +697,14 @@ public Object[] getRecordKey(int ordinal) {
         return results;
     }
 
+    public long approxHeapFootprintInBytes() {
+        PrimaryKeyIndexHashTable state = hashTableVolatile;
+        if (state == null) {
+            return 0;
+        }
+        return state.hashTable.approxHeapFootprintInBytes();
+    }
+
     /**
      * @param ordinal ordinal of root object
      * @param field   field to traverse