propagateSymbols(handler, nrHandler) {
    for (const symbol of Object.getOwnPropertySymbols(handler)) {
      logger.trace(`Setting symbol ${symbol.toString()} on handler`)
      nrHandler[symbol] = handler[symbol]
    }
  }
Response-streaming handlers are identified by symbol properties on the function.
We propagate any symbols if they're present, so that the handler keeps its signatue for any AWS features that rely on symbols
@param handler
@param nrHandler