Sidekiq metrics server should export own metrics

In #343759 (closed) we are moving the SidekiqExporter to a separate server process.

This means that any resource use caused by this class and its dependencies are now "invisible" since they move out of the worker processes (which are exporting their own metrics) and into the as-of-now unmonitored parent process.

With the sidekiq-cluster parent process as well as the metrics server itself becoming more important to overall system observability, we should probably also export metrics about them, too.