R-Precision variant of `Precision` metrics

[blondered]

Feature Description

Add r_precision: bool = False parameter to Precision metric. When r_precision is set to True, number of True Positives in K recommendations for user are divided by min(K, test items count for user) instead of simple division by K.

Why this feature?

This variant has better interpretation. https://www.shaped.ai/blog/evaluating-recommendation-systems-part-1

Additional context

No response