# File lib/new_relic/agent/instrumentation/curb.rb, line 152
      def install_header_callback( request, wrapped_response )
        original_callback = request.on_header
        request._nr_original_on_header = original_callback
        request._nr_header_str = ''
        request.on_header do |header_data|
          wrapped_response.append_header_data( header_data )

          if original_callback
            original_callback.call( header_data )
          else
            header_data.length
          end
        end
      end