Don't clean Sidekiq processes when fetching metrics

The Sidekiq process metrics API endpoint uses the default argument for
`clean_plz` to `Sidekiq::ProcessSet`, which is `true`. We don't actually
need to clean the processes here because it's a read-only API endpoint,
and Sidekiq does this automatically in the regular processes.

lib/api/sidekiq_metrics.rb

@@ -17,7 +17,7 @@ module API
       end
 
       def process_metrics
-        Sidekiq::ProcessSet.new.map do |process|
+        Sidekiq::ProcessSet.new(false).map do |process|
           {
             hostname:    process['hostname'],
             pid:         process['pid'],