prometheus adapter ApiService for custom and external metrics

[Deepzima]

The prometheus adapter currently does not create external and custom metrics for HPA

In the implementation, we only define the possibility of using the most common metrics based on CPU and mem. Still, in the readme, we describe the possibility of using aggregate metrics based on other resource types Kubernetes based(custom) and external or not of type kubernetes(external) :
The Prometheus adapter provides an implementation of Kubernetes
resource metrics,
custom metrics, and
external metrics APIs.

[nutellinoit]

#157 merged, closing issue as completed