Only enable unicorn metrics when not in sidekiq mode.

+ remove rebased test case

config/initializers/7_prometheus_metrics.rb

@@ -29,6 +29,9 @@ Sidekiq.configure_server do |config|
 end
 
 if Gitlab::Metrics.prometheus_metrics_enabled?
+  unless Sidekiq.server?
     Gitlab::Metrics::Samplers::UnicornSampler.initialize_instance(Settings.monitoring.unicorn_sampler_interval).start
+  end
+
   Gitlab::Metrics::Samplers::RubySampler.initialize_instance(Settings.monitoring.ruby_sampler_interval).start
 end

spec/lib/gitlab/metrics/sidekiq_middleware_spec.rb

@@ -21,10 +21,6 @@ describe Gitlab::Metrics::SidekiqMiddleware do
     let(:test_worker_class) { double(:class, name: 'TestWorker') }
     let(:worker) { double(:worker, class: test_worker_class) }
 
-    it 'reports correct action based on worker class' do
-
-    end
-
     it 'tracks the transaction' do
       run(worker, message)
     end