fix grpc: migrate to new unary RPC API for clients (#3)

Author: Anton3

src/hello.cpp

@@ -46,7 +46,7 @@ std::string SayHelloTo(std::string_view name, UserType type) {
       return fmt::format("Hi again, {}!\n", name);
   }
 
-  UASSERT(false);
+  UINVARIANT(false, "Invalid user type");
 }
 
 void AppendHello(userver::components::ComponentList& component_list) {

src/hello_client.cpp

@@ -10,20 +10,8 @@ std::string HelloClient::SayHello(std::string name) {
   handlers::api::HelloRequest request;
   request.set_name(std::move(name));
 
-  // Deadline must be set manually for each RPC
-  auto context = std::make_unique<grpc::ClientContext>();
-  context->set_deadline(
-      userver::engine::Deadline::FromDuration(std::chrono::seconds{20}));
-
-  // Initiate the RPC. No actual actions have been taken thus far besides
-  // preparing to send the request.
-  auto stream = client_.SayHello(request, std::move(context));
-
-  // Complete the unary RPC by sending the request and receiving the response.
-  // The client should call `Finish` (in case of single response) or `Read`
-  // until `false` (in case of response stream), otherwise the RPC will be
-  // cancelled.
-  handlers::api::HelloResponse response = stream.Finish();
+  // Perform RPC by sending the request and receiving the response.
+  auto response = client_.SayHello(request);
 
   return std::move(*response.mutable_text());
 }