Draft: Export metrics for sidekiq-cluster metrics server

What does this MR do and why?

See #344629 (closed)

Based on !73049 (closed)

Now that we moved the metrics server into a separate process running as part of sidekiq-cluster, we want to export metrics about this new Ruby process, too, so as not to leave a gap in observability.

We're starting simple by exporting everything that is collected by RubySampler, which are important for resource utilization, especially memory use.

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Related to #344629 (closed)