If you are talking about http://stats.wikimedia.org/v2 the metric definition has not changed from the (now-called)  "legacy wikistats 1" (http://stats.wikimedia.org) . In the V2 system metrics are surfaced over a new UI and also new APIs so they are available programatically.

Some docs:

Metric definition: https://wikitech.wikimedia.org/wiki/Analytics/AQS/Wikistats_2/Metrics_Definition

Vetting of calculations metrics using the "legacy"metrics as baseline: https://wikitech.wikimedia.org/wiki/Analytics/AQS/Wikistats_2/Data_Quality

The edit metrics API: https://wikitech.wikimedia.org/wiki/Analytics/AQS/Wikistats_2

The UI: https://wikitech.wikimedia.org/wiki/Analytics/Systems/Wikistats_2

Thanks,

Nuria