code: public Credential refreshCredential(Credential credential) throws IOException {
      if (isCredentialRefreshable(credential)) {
        if (!callRefreshToken(credential)) {
          libLogger.warn("OAuth2 token could not be refreshed. "
              + "Add a refreshListener to the Credential to capture this failure.");
        }
      }
      return credential;
    }
docs: Refreshes the credential if within the refresh window.
  
  @throws IOException if there was an problem refreshing the token