WIP - Add kcp_cluster_index_clusters_on_shard metric

[xrstf]

Summary

This PR adds a new metric to the cluster index that reports the number of known logicalclusters per shard.

What Type of PR Is This?

/kind feature

Release Notes

Add new `kcp_cluster_index_clusters_on_shard` metric that contains the number of known logicalclusters per shard (metric has a `shard` label).

[kcp-ci-bot]

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@xrstf: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-kcp-test-e2e-shared	9df7cfc	link	true	/test pull-kcp-test-e2e-shared
pull-kcp-test-e2e-sharded	9df7cfc	link	true	/test pull-kcp-test-e2e-sharded
pull-kcp-test-e2e-multiple-runs	9df7cfc	link	true	/test pull-kcp-test-e2e-multiple-runs

Full PR test history

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[embik]

Suggested change

	Namespace: "kcp",
	Subsystem: "cluster_index",
	Name: "clusters_on_shard",
	Help: "Number of known logicalclusters per shard.",
	Namespace: "kcp",
	Subsystem: "front_proxy",
	Name: "indexed_clusters",
	Help: "Number of logicalclusters indexed by kcp-front-proxy or embedded localproxy per shard.",

[xrstf]

I first had "front_proxy" in there, but since the metric is available also on shards (because the localproxy also uses an index), it felt very confusing to name the metric "kcp_front_proxy_something_something".