Create a separate metrics server for Sidekiq

Move metrics exporter logic to a separate process.

With sidekiq-cluster we run a lightweight parent process that supervises all Sidekiq workers. Instead of workers racing to bind the exporter port, we should consider moving SidekiqExporter into a new process instead, which will be supervised by sidekiq-cluster just as workers are now. This will provide better fault and resource isolation.

The metrics server will be responsible for serving metrics on a separate port as the workers (e.g. localhost:3907/metrics)