modified-pubsub-client.go

jbts, err := json.Marshal(PubSubKey) // assuming the data of the json key is in the PubSubKey struct
	if err != nil {
		log.Println("could not convert api json to bytes:", err)
		return
	}
	conf, err := google.JWTConfigFromJSON(jbts, "https://www.googleapis.com/auth/pubsub")
	if err != nil {
		log.Fatal(err)
	}
	ts := conf.TokenSource(ctx)
	client, err := pubsub.NewClient(ctx, config.ProjectID, option.WithTokenSource(ts))