Disable Unicorn sampling in Sidekiq since there are no Unicorn sockets to monitor

This reduces some polling overhead and reduces the need to aggregate
empty metrics from the Sidekiq worker.

changelogs/unreleased/sh-disable-unicorn-sampling-sidekiq.yml

+---
+title: Disable Unicorn sampling in Sidekiq since there are no Unicorn sockets to monitor
+merge_request:
+author:
+type: performance

config/initializers/8_metrics.rb

@@ -123,7 +123,9 @@ def instrument_classes(instrumentation)
 end
 # rubocop:enable Metrics/AbcSize
 
-Gitlab::Metrics::UnicornSampler.initialize_instance(Settings.monitoring.unicorn_sampler_interval).start
+unless Sidekiq.server?
+  Gitlab::Metrics::UnicornSampler.initialize_instance(Settings.monitoring.unicorn_sampler_interval).start
+end
 
 Gitlab::Application.configure do |config|
   # 0 should be Sentry to catch errors in this middleware