promql/engine.go
func (ev *evaluator) errorf(format string, args ...interface{}) {
	ev.error(errors.Errorf(format, args...))
}
func (ev *evaluator) errorf(format string, args ...interface{}) {
	ev.error(errors.Errorf(format, args...))
}
// errorf causes a panic with the input formatted into an error.