code: func New() *http.Client {
  	cli := cleanhttp.DefaultPooledClient()
  	cli.Transport = &userAgentRoundTripper{
  		userAgent: UserAgentString(),
  		inner:     cli.Transport,
  	}
  	return cli
  }
docs: // New returns the DefaultPooledClient from the cleanhttp
  // package that will also send a Terraform User-Agent string.