function marshallCustomIdentifiers(request, shape) {
	  var identifiers = {};
	  marshallCustomIdentifiersHelper(identifiers, request.params, shape);
	  return identifiers;
	}
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