[errors] Greatly simplify common/errors package.

grpc/grpcutil/errors.go

Index: grpc/grpcutil/errors.go
diff --git a/grpc/grpcutil/errors.go b/grpc/grpcutil/errors.go
index e34f3216a004f95904eaa54abe83ec1a9afdac76..e589ac37091e91ae273b39e674486843be2985b8 100644
--- a/grpc/grpcutil/errors.go
+++ b/grpc/grpcutil/errors.go
@@ -89,7 +89,9 @@ func WrapIfTransient(err error) error {
 
 type grpcCodeTag struct{ Key errors.TagKey }
 
-func (g grpcCodeTag) With(code codes.Code) errors.TagValue { return errors.MkTagValue(g.Key, code) }
+func (g grpcCodeTag) With(code codes.Code) errors.TagValue {
+	return errors.TagValue{Key: g.Key, Value: code}
+}
 func (g grpcCodeTag) In(err error) (v codes.Code, ok bool) {
 	d, ok := errors.TagValueIn(g.Key, err)
 	if ok {