Implement a statistics HTTP server

Lots of repeated code here.

What about extracting a common helper function - 

     template <typename Type>
    std::make_unique<MetricsResourceList> makeResource (MetricsImpl& metrics)
    {
        return {metrics_store_cast<Type> (metrics.getMetricStore<Type> ())};
    }

or - shouldn't getMetricStore change to be like that helper function?
    }