* Support getting certificate information from a kubeconfig file
* Support relative paths for cluster CA and user certificate in kubeconfig
* Determine relative using filepath.IsAbs
* Make relative path logic actually work, add test. Move all kubeconfig parsing into parsing specific function
The existing implementation consists of a collector that exports
information from a tls.ConnectionState returned by the prober function.
This won't necessarily integrate well with additional probers that
retrieve certs from sources other than a tls handshake (from file, for
instance).
I've made the probing more generically expandable by removing the
collector and instead registering and collecting metrics inside the
prober. This makes it possible to collect the same metrics in a
different way, or collect different metrics depending on the prober.
