Add project_id, location, and cluster labels via relabeling to kubelet metrics

[pintohutch]

The hardcoded scrape_config for the kubelet and does not include project_id, location, or cluster, which is in contrast to the scrape_config relabeling from PodMonitoring.

In practice, this isn't a big deal as those labels are injected in-memory by the export logic in the collectors from the --export.labels.* flags, which are upserted by the operator at runtime.

But to avoid relying on that behavior, and for consistency with PodMonitoring, we should relabel before ingesting to the collector.