replace `jsonpb` into `protojson`

Replacing `jsonpb` into `protojson` is needed because `jsonpb` has been deprecated.

Additionally, protojson allows us to use marshalOptions to use `snake_case` keys for marshalling. Only `camelCase` is supported currently in grpc access logs and it seems it's not a good practice.

```
protojson.MarshalOptions{
    UseProtoNames:   true,
    EmitUnpopulated: true,
},
```
