Address review comments

Change-Id: Ie24e59a43eeca52075465f930d269de7482aaadc

Author: trollyxia

google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/data/v2/internal/AbstractProtoStructReader.java

@@ -357,7 +357,8 @@ Object decodeValue(Value value, SqlType<?> type) {
         return ProtoStruct.create(schema, value.getArrayValue());
       case PROTO:
         try {
-          return ((SqlType.Proto<?>) type).getParserForType().parseFrom(value.getBytesValue());
+          SqlType.Proto protoType = (SqlType.Proto) type;
+          return protoType.getParserForType().parseFrom(value.getBytesValue());
         } catch (InvalidProtocolBufferException e) {
           throw new IllegalStateException("Unable to parse value to proto " + type, e);
         }

google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/data/v2/models/sql/SqlType.java

@@ -143,6 +143,7 @@ interface Field extends ColumnMetadata {
    *
    * @param <T> Java type of the protobuf message
    */
+  @BetaApi("This feature is currently experimental and can change in the future")
   interface Proto<T extends AbstractMessage> extends SqlType<T> {
 
     /**
@@ -158,6 +159,7 @@ interface Proto<T extends AbstractMessage> extends SqlType<T> {
    *
    * @param <T> Java type of the protobuf enum
    */
+  @BetaApi("This feature is currently experimental and can change in the future")
   interface Enum<T extends ProtocolMessageEnum> extends SqlType<T> {
 
     /**