add json codec for grpc (#1908)

transport/grpc/codec.go

@@ -0,0 +1,36 @@
+package grpc
+
+import (
+	"fmt"
+
+	"github.com/go-kratos/kratos/v2/encoding/json"
+	"google.golang.org/grpc/encoding"
+	"google.golang.org/protobuf/proto"
+)
+
+func init() {
+	encoding.RegisterCodec(codec{})
+}
+
+// codec is a Codec implementation with protobuf. It is the default codec for gRPC.
+type codec struct{}
+
+func (codec) Marshal(v interface{}) ([]byte, error) {
+	vv, ok := v.(proto.Message)
+	if !ok {
+		return nil, fmt.Errorf("failed to marshal, message is %T, want proto.Message", v)
+	}
+	return json.MarshalOptions.Marshal(vv)
+}
+
+func (codec) Unmarshal(data []byte, v interface{}) error {
+	vv, ok := v.(proto.Message)
+	if !ok {
+		return fmt.Errorf("failed to unmarshal, message is %T, want proto.Message", v)
+	}
+	return json.UnmarshalOptions.Unmarshal(data, vv)
+}
+
+func (codec) Name() string {
+	return "json"
+}