Update unit test to use delegatesTo() with the mocks

xds/src/test/java/io/grpc/xds/XdsClientMetricReporterImplTest.java

@@ -17,6 +17,7 @@
 package io.grpc.xds;
 
 import static com.google.common.truth.Truth.assertThat;
+import static org.mockito.AdditionalAnswers.delegatesTo;
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.ArgumentMatchers.argThat;
 import static org.mockito.ArgumentMatchers.eq;
@@ -82,14 +83,14 @@ public class XdsClientMetricReporterImplTest {
   @Rule
   public final MockitoRule mocks = MockitoJUnit.rule();
 
-  @Mock
-  private MetricRecorder mockMetricRecorder;
   @Mock
   private XdsClient mockXdsClient;
-  @Mock
-  private BatchRecorder mockBatchRecorder;
   @Captor
   private ArgumentCaptor<BatchCallback> gaugeBatchCallbackCaptor;
+  private MetricRecorder mockMetricRecorder = mock(MetricRecorder.class,
+      delegatesTo(new MetricRecorderImpl()));
+  private BatchRecorder mockBatchRecorder = mock(BatchRecorder.class,
+      delegatesTo(new BatchRecorderImpl()));
 
   private XdsClientMetricReporterImpl reporter;
 
@@ -372,6 +373,12 @@ public boolean matches(T instrument) {
     });
   }
 
+  static class MetricRecorderImpl implements MetricRecorder {
+  }
+
+  static class BatchRecorderImpl implements BatchRecorder {
+  }
+
   static class TestlogHandler extends Handler {
     List<LogRecord> logs = new ArrayList<>();