从GCP发布/订阅中捕获错误代码

I am using go package for pub/sub. On my API dashboard I see this error(google.pubsub.v1.Subscriber.StreamingPull - error code 503). Per docs(https://cloud.google.com/pubsub/docs/reference/error-codes) it seems it is transient condition but better to implement backoff strategy(https://cloud.google.com/storage/docs/exponential-backoff). the question is I am not able to wrap my head where this error code is coming on Receive method.

Here is func:

err = sub.Receive(ctx, func(ctx context.Context, m *pubsub.Message) {
        // Dump message
        // log.Printf("Got message: %s", m.Data)

        // Decoding coming message
        err = json.NewDecoder(bytes.NewReader(m.Data)).Decode(&msg)
        if err != nil {
            log.Printf("Error decoding - %v", err)
        }

        // See streaming messages
        log.Printf(" %s : %s : Product updated for Product Id(%d) : Product Title(%s)",
            msg.AuthID,
            msg.TraceID,
            msg.Product.ID,
            msg.Product.Title,
        )

        //
        // Some business logic
        //


        // Acknowledge on recieve method
        m.Ack()
    })

    if err != context.Canceled {
        // if err != nil {
        return errors.Wrap(err, "Error occurred on recieve data from topic: blah")
    }