Embed "Unimplemented" structs as proto recommended

Embedding these structs will be mandatory after migratring off from
gogo/protobuf (see #6564).

Signed-off-by: Kazuyoshi Kato <katokazu@amazon.com>

services/introspection/service.go

@@ -62,6 +62,7 @@ func init() {
 
 type server struct {
 	local api.IntrospectionClient
+	api.UnimplementedIntrospectionServer
 }
 
 var _ = (api.IntrospectionServer)(&server{})