The `root_ca_cert_publisher_sync_duration_seconds` metric tracks the sync
duration in the root CA cert publisher per code and namespace. In
clusters with a high namespace turnover (like CI clusters), this may
cause the kube-controller-manager to expose over 100k series to
Prometheus, which may cause degradation of that service.
Drop the `namespace` label to remove the metrics' cardinality, tracking
this metric by namespace does not justify the impact of keeping it.
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