Add Average Precision and R2 to the metrics

[thibaultdvx]

Hi,
I would like to suggest to add the Average Precision in the classification metrics. I think that the implementation would be very similar to ROC AUC, so it wouldn't be a big deal to integrate it.
I would be happy to make a PR to do so.

I also feel that an important regression metric is missing: R2 score. Is there any reason why?
Once again, I would be happy to help to implement it!

WDYT?

[KumoLiu]

Hi @thibaultdvx, Welcome, and thank you for your contribution! For more details and guidance, please refer to the CONTRIBUTION GUIDE.