Separate incoming and outgoing metadata in context

This will prevent the incoming RPCs' metadata from appearing in outgoing RPCs
unless it is explicitly copied, e.g.:

incomingMD, ok := metadata.FromContext(ctx)
if ok {
  ctx = metadata.NewContext(ctx, incomingMD)
}

Fixes #1148

grpclb/grpclb_test.go

@@ -215,7 +215,7 @@ type helloServer struct {
 }
 
 func (s *helloServer) SayHello(ctx context.Context, in *hwpb.HelloRequest) (*hwpb.HelloReply, error) {
-	md, ok := metadata.FromContext(ctx)
+	md, ok := metadata.FromIncomingContext(ctx)
 	if !ok {
 		return nil, grpc.Errorf(codes.Internal, "failed to receive metadata")
 	}