docs: function createReqIdHeaders(opts) {
  
      assert.object(opts, 'opts');
      assert.arrayOfString(opts.headers, 'opts.headers');
  
      var headers = opts.headers.concat(DEFAULT_HEADERS);
  
      return function reqIdHeaders(req, res, next) {
  
          for (var i = 0; i < headers.length; i++) {
              var val = req.header(headers[i]);
  
              if (val) {
                  req.id(val);
                  break;
              }
          }
  
          return next();
      };
  }