refactor discord command span in its own middleware

cmd/themis-server/main.go

@@ -785,7 +785,7 @@ func registerHandlers(sess *discordgo.Session, handlers map[string]Handler) {
 			defer cancel()
 
 			if h, ok := handlers[i.ApplicationCommandData().Name]; ok {
-				withLogging(i.ApplicationCommandData().Name, h)(ctx, s, i)
+				inSpan(i.ApplicationCommandData().Name, withLogging(i.ApplicationCommandData().Name, h))(ctx, s, i)
 			}
 		case discordgo.InteractionModalSubmit:
 			ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
@@ -952,11 +952,17 @@ func serve(address string) error {
 	return http.ListenAndServe(address, nil)
 }
 
-func withLogging(name string, h Handler) Handler {
+func inSpan(name string, h Handler) Handler {
 	return func(ctx context.Context, s *discordgo.Session, i *discordgo.InteractionCreate) error {
 		ctx, span := tracer.Start(ctx, fmt.Sprintf("discord_command %s", name))
 		defer span.End()
 
+		return h(ctx, s, i)
+	}
+}
+
+func withLogging(name string, h Handler) Handler {
+	return func(ctx context.Context, s *discordgo.Session, i *discordgo.InteractionCreate) error {
 		start := time.Now()
 		logCommandInvocation(ctx, name, s, i)
 		err := h(ctx, s, i)