0.9.0: Support Prometheus official client multi-process mode

It is needed when you can't scrape every Sidekiq process directly and want to aggregate metrics from all worker processes on a server and expose them from within a single dedicated process.

Added

• Configuration setting to declare worker in-process metrics outside workers.

  It can be needed for official Prometheus client in multi-process mode where separate process expose metrics from Sidekiq worker processes.

• most_recent aggregation for all cluster-wide gauges.

  It is also needed for official Prometheus client in multi-process mode to reduce number of time series.