如何使用protobuf构建基于键的搜索gRPC调用

Question

a question about gRPC with protobuf. I'm essentially building a set of microservices and we use gRPC and protocol buffer version 3, language used is Go. Everything works well in most of the cases, however we have some rpc functions planned which should perform a search based on a set of keys. The issue we have is with say "optional" keys. Let's say we have a message with search keys like name,surname, location. IN this case if they are defaulted to empty string I can assume they don't have to be included in the search so in case i.e. we're using mongodb, we just use omitempty tag to exclude empty fields from the search. IN case of things like "age" we might assume that 0 age is null...even though it might be wrong in general. Furthermore if the key is of a boolean type: "Active" i.e., we'd like to search for specific values: true, folse...or to exclude it....but we don't have null in protobuf. Has anybody any idea about how to structure the call and the message in these cases?