if the message has an optional field with a return value, the proto will attach a same name field with `_` prefix. why?

### Problem description
if the message has an option field with a return value, the proto will attach a same name field with `_` prefix. why?

### Reproduction steps
- message like this
```js
message Res {
uint32 code = 1;
optional string msg = 2;
string status = 3;
optional string error = 4;
optional uint32 count = 5;
}
```
- server retun

- real result client got

### Environment
- OS name, version and architecture: windows11
- Node version : 16.15
- Node installation method. binary
- If applicable, compiler version [e.g. clang 3.8.0-2ubuntu4]
- Package name and version: 1.10.8

### Additional context