Often prometheus is gated by some proxy requiring an auth bearer
token. Currently there is no possibility to configure one except for
providing a full-fledged kubeconfig.
This fixes it by adding a new flag pointing to an optional file
containing the auth bearer for prometheus communication.
This disables the custom metrics API when no custom metrics rules are
given (the resource metrics API acts equivalently). This allows a given
adapter to serve only one of the APIs, if desired.
This introduces support for the resource metrics in the adapter.
The individual queries and window sizes are fully customizable via a new
config section. This uses just the generic machinery from
metrics-server to serve the API.
