Add some comments

src/main/java/com/wechi/adweb/bridge/google/analytics/GADataService.java

@@ -22,6 +22,11 @@ import java.io.IOException;
 import java.util.List;
 import java.util.Objects;
 
+/**
+ * @author wfansh
+ *     <p>See https://developers.google.com/analytics/devguides/reporting/data/v1/api-schema for GA
+ *     metrics and dimensions.
+ */
 @Slf4j
 @Service
 public class GADataService {