code: def credentials(credentials_override = nil)
        result = super(credentials_override)
        validate_headers_for_server(result)
  
        extra_headers = {
          'userAgent' => generate_user_agent(),
          'developerToken' => result[:developer_token]
        }
        extra_headers['clientCustomerId'] = result[:client_customer_id] if
            result[:client_customer_id]
        extra_headers['validateOnly'] = 'true' if @validate_only
        extra_headers['partialFailure'] = 'true' if @partial_failure
        result[:extra_headers] = extra_headers
        return result
      end
docs: Create the list of credentials to be used by the auth handler for header
   generation.