Some reverse proxies do not support overriding 'Location' header
in 3xx responses. This commit adds a --url_base_prefix option
that adds a prefix to all URLs of cAdvisor so that
redirects go to the correct places.
This change generalizes the existing ContainerNameToLabelsFunc to allow the user to fully control all labels attached to exported Prometheus metrics. The existing behavior is available as DefaultContainerLabelsFunc and is used if no custom function is provided.
This will allow Kubernetes to filter out its internal Docker labels.