Extend configuration for exported labels or metrics without labels

Hello,

would it be possible to implement extended configuration so that some of metrics are not subject of label injections? Such as `kube_node_labels` do not use namespaces. So such metrics could be whitelisted in configuration.

Or there are some metrics that use `exported_namespace` instead of `namespace`. Could be such metrics listed so that label injector injects the `exported_namespace` instead of the `namespace`? Thanks.

Start by locating the label injector and existing configuration for namespace labels, then inspect handling of kube_node_labels and metrics using exported_namespace. Done means configuration can whitelist metrics without namespace injection and select exported_namespace where required, with tests covering both cases.

Written by the indexing model from the issue text.