Add recording rules that will be federated by aggregate prometheus and garden-prometheus

[ialidzhikov]

How to categorize this PR?

/area monitoring
/kind enhancement

What this PR does / why we need it:
This PR adds recording rules to federate the registry_proxy_pushed_bytes_total, registry_proxy_pulled_bytes_total metrics in the Seed cluster's aggregate prometheus and in then also in the prometheus in the runtime cluster.

The motivation for this PR is that currently it is not possible to fetch check the above-mentioned metrics for several clusters. We only have the Registry Cache dashboard and we display the "delta" (pushed to clients - pulled from upstream) there. However, to conclude on the improvements by the registry-cache extensions, it would be nice to be able to fetch the metrics for given set of Shoots. For example, to be able to do queries like: fetch the "delta" for all Shoots in given Project, or fetch the "delta" for all Shoots in the landscape.

Which issue(s) this PR fixes:
Part of #3

Special notes for your reviewer:
These are the recording rules that are present in the aggregate prometheus:

A prometheus query that can be used to fetch the "delta":

  (sum_over_time((shoot:registry_proxy_pushed_bytes_total:sum)[1d:1m]) * 60)
-
  (sum_over_time((shoot:registry_proxy_pulled_bytes_total:sum)[1d:1m]) * 60)

Many thanks to @istvanballok for the help with this task.

Release note:

The registry-cache extension defines recording rules (`shoot:registry_proxy_pushed_bytes_total:sum` and `shoot:registry_proxy_pulled_bytes_total:sum`) that are federated in the Seed cluster's aggregate prometheus and also in the prometheus in the runtime cluster. These rules make possible to query registry-cache related metrics from the prometheus in the runtime cluster and in this way get an overview for given set of Shoot clusters.

[dimitar-kostadinov]

/lgtm

[gardener-prow]

LGTM label has been added.

Git tree hash: 4288ae133e9e909b9733c559c951b4426d9871fd

[gardener-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dimitar-kostadinov

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