Add an option to detect Vec metric without specifying labels

[yeya24]

If we want to use NewXXXVec to create metrics Vec, we need to specify labels. So if labels are not specified, then we can detect it and that's better to use just plain metric type.

However, this might be not easy to implement. We can use any type of identifiers for the label slice, even using objs from other packages, so it is tricky.

s := []string{"test"}

prometheus.NewCounterVec(prometheus.CounterOpts{}, s)