use proto3 syntax in c++ test protos

test/cpp/util/echo.proto

@@ -28,7 +28,7 @@
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-syntax = "proto2";
+syntax = "proto3";
 
 import "test/cpp/util/messages.proto";
 

test/cpp/util/echo_duplicate.proto

@@ -30,7 +30,7 @@
 
 // This is a partial copy of echo.proto with a different package name.
 
-syntax = "proto2";
+syntax = "proto3";
 
 import "test/cpp/util/messages.proto";
 

test/cpp/util/messages.proto

@@ -28,32 +28,32 @@
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-syntax = "proto2";
+syntax = "proto3";
 
 package grpc.cpp.test.util;
 
 message RequestParams {
-  optional bool echo_deadline = 1;
+  bool echo_deadline = 1;
-  optional int32 client_cancel_after_us = 2;
+  int32 client_cancel_after_us = 2;
-  optional int32 server_cancel_after_us = 3;
+  int32 server_cancel_after_us = 3;
-  optional bool echo_metadata = 4;
+  bool echo_metadata = 4;
-  optional bool check_auth_context = 5;
+  bool check_auth_context = 5;
-  optional int32 response_message_length = 6;
+  int32 response_message_length = 6;
-  optional bool echo_peer = 7;
+  bool echo_peer = 7;
 }
 
 message EchoRequest {
-  optional string message = 1;
+  string message = 1;
-  optional RequestParams param = 2;
+  RequestParams param = 2;
 }
 
 message ResponseParams {
-  optional int64 request_deadline = 1;
+  int64 request_deadline = 1;
-  optional string host = 2;
+  string host = 2;
-  optional string peer = 3;
+  string peer = 3;
 }
 
 message EchoResponse {
-  optional string message = 1;
+  string message = 1;
-  optional ResponseParams param = 2;
+  ResponseParams param = 2;
 }