Sending metadata
Another feature that builds upon contexts is the possibility to pass metadata through calls. In gRPC, these metadata can be HTTP headers or HTTP trailers. They are both a list of key-value pairs that are used for many purposes such as passing authentication tokens and digital signatures, data integrity, and so on. In this section, we are mostly going to focus on sending metadata through headers. Trailers are simply headers that are sent after a message and not before. They are less used by developers but are used by gRPC to implement streaming interfaces. Anyway, if you are interested, you can look at the grpc.SetTrailer function (https://pkg.go.dev/google.golang.org/grpc#SetTrailer).
Our use case will be to pass an auth token to the UpdateTasks RPC endpoint, and after checking it, we will decide to either update the task or return an Unauthenticated error. Obviously, we are not going to deal with how to generate the auth token because this is an implementation...
