title: how the retryRequest method in ServiceUnavailableRetryStrategy is called?
body: <p>I have a question regarding the <code>retryRequest</code> method in <code>ServiceUnavailableRetryStrategy</code>, how this method is called?</p>
  
  <p>We impelement the interface <code>ServiceUnavailableRetryStrategy</code>, and override the <code>retryRequest</code>, then how this method is called? Do we need to call it explicitly?</p>
  
  <p>Thank you!</p>