code: function marshallCustomIdentifiers(request, shape) {
    var identifiers = {};
    marshallCustomIdentifiersHelper(identifiers, request.params, shape);
    return identifiers;
  }
docs: Get custom identifiers for cache key.
  Identifies custom identifiers by checking each shape's `endpointDiscoveryId` trait.
  @param [object] request object
  @param [object] input shape of the given operation's api
  @api private