Investigate potential bug in billing.

It’s related to stats and billing of Backend metrics. It seems we missed https://github.com/3scale/porta/blob/704cd10aa676dfd609a9104843361d165cb99380/app/lib/stats/base.rb#L116.

It needs to be changed to all_metrics, otherwise variable cost won’t work for Backend metrics. That is, System won’t bill variable cost of application plan pricing rules defined upon backend api metrics.