Partial delete metrics

[davidcollom]

This is part of https://github.com/jetstack/version-checker/issues/147

In an attempt to resolve the Old Container metrics, as the current delete assumes that we know the exact set of labels, including current tag and latest tag.. if the latest of current tags change... this could cause issues.

Therefore Attempting to delete the metrics based of the container, pod and namespace could be a more reliable way of resolving this.