In the KEP 1710 we promised to have all SELinux metrics with access mode
label, so cluster admin is able to distinguish when RWOP volumes are
failing to mount (-> SELinuxMountReadWriteOncePod feature gate must be
disabled) or volumes with any other access modes are failing (->
SELinuxMount feature gate must be disabled).
Adding the label to kubelet is quite straightforward, there were some
changes needed in the e2e test. Now grabMetrics() collects values of all
SELinux related metrics with all labels. It only skips unrelated volume
plugins. And waitForMetricIncrease gets metric with all labels on input, so
it can check that say RWOP metric increased and RWX one did not.
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