test: fix type in test (#1653)

Author: mutianf

google-cloud-bigtable/src/test/java/com/google/cloud/bigtable/data/v2/it/BuiltinMetricsIT.java

@@ -32,14 +32,14 @@
 import com.google.monitoring.v3.TimeInterval;
 import com.google.protobuf.util.Timestamps;
 import java.io.IOException;
-import java.time.Duration;
 import java.util.ArrayList;
 import org.junit.AfterClass;
 import org.junit.BeforeClass;
 import org.junit.ClassRule;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 import org.junit.runners.JUnit4;
+import org.threeten.bp.Duration;
 
 @RunWith(JUnit4.class)
 public class BuiltinMetricsIT {