IVFVectorsFormat doesn't report off-heap stats at the moment (#130553)

Author: thecoop

server/src/test/java/org/elasticsearch/index/codec/vectors/IVFVectorsFormatTests.java

@@ -38,6 +38,8 @@
 import static java.lang.String.format;
 import static org.elasticsearch.index.codec.vectors.IVFVectorsFormat.MAX_VECTORS_PER_CLUSTER;
 import static org.elasticsearch.index.codec.vectors.IVFVectorsFormat.MIN_VECTORS_PER_CLUSTER;
+import static org.hamcrest.Matchers.anEmptyMap;
+import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.is;
 import static org.hamcrest.Matchers.oneOf;
 
@@ -83,6 +85,25 @@ protected Codec getCodec() {
         return TestUtil.alwaysKnnVectorsFormat(format);
     }
 
+    @Override
+    protected void assertOffHeapByteSize(LeafReader r, String fieldName) throws IOException {
+        var fieldInfo = r.getFieldInfos().fieldInfo(fieldName);
+
+        if (r instanceof CodecReader codecReader) {
+            KnnVectorsReader knnVectorsReader = codecReader.getVectorReader();
+            if (knnVectorsReader instanceof PerFieldKnnVectorsFormat.FieldsReader fieldsReader) {
+                knnVectorsReader = fieldsReader.getFieldReader(fieldName);
+            }
+            var offHeap = knnVectorsReader.getOffHeapByteSize(fieldInfo);
+            long totalByteSize = offHeap.values().stream().mapToLong(Long::longValue).sum();
+            // IVF doesn't report stats at the moment
+            assertThat(offHeap, anEmptyMap());
+            assertThat(totalByteSize, equalTo(0L));
+        } else {
+            throw new AssertionError("unexpected:" + r.getClass());
+        }
+    }
+
     @Override
     public void testAdvance() throws Exception {
         // TODO re-enable with hierarchical IVF, clustering as it is is flaky