Fix missing metrics when querying the stats API and filtering by a custom property

When querying the stats api, specifically the timeseries endpoint, you are currently unable to get all metrics returned if you filter by a custom property.

Although you can retrieve pageviews and visitors, most critically for us the visits metric is missing / unable to be requested.

All these core metrics are available in the front end dashboards when filtered by custom properties and we wrongly assumed that the api would mirror this :(