type endpointLister interface {
    List(ctx context.Context) ([]InstanceMetadata, error)
    GetLoadBalancerEndpoint(ctx context.Context) (host, port string, err error)
}

func JoinServiceEndpoints(ctx context.Context, lister endpointLister) ([]string, error) {
    lbIP, _, err := lister.GetLoadBalancerEndpoint(ctx)
    if err != nil {
        // Maybe do additional error handling here in case of no loadbalancer (ignore error?)
        return nil, fmt.Errorf("retrieving load balancer endpoint from cloud provider: %w", err)
    }
    // Old JoinServiceEndpoints code
    joinEndpoints := []string{}
    ...

    // Return all endpoints, with LB as the first endpoint
    return append(net.JoinHostPort(lbEndpoint, strconv.Itoa(constants.JoinServiceNodePort)), joinEndpoints), nil
}