Document scrape_samples_post_metric_relabeling and scrape_samples_scraped
Document scrape_samples_post_metric_relabeling and scrape_samples_scraped

Fixes #614
Fixes #679

ddfreyne/prometheus-docs,ddfreyne/prometheus-docs,prometheus/docs,chosenken/docs,chosenken/docs,chosenken/docs,du2016/prometheus-docs,ddfreyne/prometheus-docs,prometheus/docs,du2016/prometheus-docs,du2016/prometheus-docs,prometheus/docs,ddfreyne/prometheus-docs,chosenken/docs