add(*): Add filedmask interceptor

[linhbkhn95]

Changes

• Add fieldmask interceptor that allow filter response base on client's request intent.

How to use

Pre-condition

You have to add field_mask into proto's request like

message Request{
  // main fields
  google.protobuf.FieldMask field_mask = 100;
}

1. install via go get

go get github.com/grpc-ecosystem/go-grpc-middleware/v2
`

2. Import and inject into grpc interceptor
   The code in your application should be like that:

import(
        // ...
        "google.golang.org/grpc"
    	fieldmaskpkg "github.com/grpc-ecosystem/go-grpc-middleware/v2/interceptors/fieldmask"


)
// ...

func main(){
    var unaryOpts []grpc.UnaryServerInterceptor{
		fieldmaskpkg.UnaryServerInterceptor(fieldmaskpkg.DefaultFilterFunc),
    }
    // Should append others interceptors
}

3. Client usage

   Protobuf definition

     message GetProductRequest{
       string id = 1;
       google.protobuf.FieldMask field_mask = 2;
     }
     message Response{
       message Result{
          repeated Product products = 1;
       }
       Result result = 1;
     }
     message Product{
        string id = 1;
        string name = 2;
        string img = 3;
        decimal price = 4;
     }
     

   Client interaction

       func GetProduct(id string) *Product{
        request := pb.GetProductRequest{
      	Id: "axaxaxx",
      	FieldMask: &fieldmaskpb.FieldMask{
      		Paths: []string{
      			"result.products.id", "result.products.name"
      		},
      	}
         }
         // ...
       }

• I have built an individual repo for this idea
  https://github.com/linhbkhn95/go-grpc-middleware-field-mask

Verification

• I wrote tests

[linhbkhn95]

@jpkrohling @bwplotka can you guys give me some suggestions in this PR?

[johanbrandhorst]

Hi, this is cool, but I'm not sure it's generically useful enough to be merged into the library. Field masks can be used for a number of different things, not just for filtering fields in a response message. I don't think we want this functionality in the library. Thank you for your PR though!

[linhbkhn95]

I know that field masks are not only for filtering responses(partial update). Can I change the package name to fit with this functionality?
@johanbrandhorst

[johanbrandhorst]

I just think the functionality is too niche to be generally useful. It's a cool example of how users can implement their own interceptors using the framework in this repo, but I don't know that it's something users will find useful otherwise. You could submit a documentation PR if you want?

[linhbkhn95]

You could submit a documentation PR if you want?. What is your mean? where is the documentation repo?

[johanbrandhorst]

I mean that we could add an example to https://github.com/grpc-ecosystem/go-grpc-middleware/tree/main/examples, something about implementing your own interceptors and use this as an example.