Percentile metrics
Generate percentiles ([0.05, 0.25, 0.5, 0.75, 0.95] for article_created, pageviews, quality_score, page_revision_count metrics
The motivation for this is initially tracking article quality in buckets of quality (e.g what percentage of articles for a given content gap category can be considered good?).
See the Quantile metrics section of the example notebook.