When we need go struct, which protobuf generated, to support the bson serialization, within the gogoprotobuf, we can use `moretags`.

However in general, the value of `bson` tag is consistent with the `json` tag, so we only need to control whether the generator automatically adds `bson` tag by a switch, which defaults to false.

Currently, three switches have been added,
1. The generator CMD parameter
```
Protoc - gofast_out = bsontag:. My. Proto
```
2. The file option
```
Option (gogoproto bsontag_all) = true;
```
3. The message option
```
Option (gogoproto bsontag) = true;
```

The bson tag will check from the command line parameter to the message option, and the settings will override the previous one.

The specific value of the bson tag can be specified by `moretags`, eg. `moretags = "bson:\"fieldName\ "`, if `moretags` is not specified, then will automatically generate value for bson tag:

  1. First get the `json` attribute in `protobuf` tag, and if it exists, use the value;
  2. If the above does not exist, the value of the `json` tag is used.

like this:

message Building {
    optional string id = 1;
    optional string name = 2;
}

message Floor {
    optional string id = 1;
    optional string name = 2;
}

message Place {
    option (gogoproto.union) = "type,typeField";
    oneof union {
        Floor floor = 1;
        Building building = 2;
    }
}

there is no change for the protobuffer, only changed for the json format. so the Place will format like this:

{
	"@type" : "Building",
        "id": "id",
	"name": "name"
}