Add sidekiq_job_completion_metric_initialize feature flag

This feature flag controls the initialisation of the
sidekiq_jobs_completion_seconds bucket when a Sidekiq server process
starts. By default it is on, but we can disable it (and restart Sidekiq)
if there is an issue.

config/feature_flags/development/sidekiq_job_completion_metric_initialize.yml

+---
+name: sidekiq_job_completion_metric_initialize
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/64637
+rollout_issue_url: https://gitlab.com/gitlab-com/gl-infra/scalability/-/issues/1133
+milestone: '14.1'
+type: development
+group: team::Scalability
+default_enabled: true

lib/gitlab/sidekiq_middleware/server_metrics.rb

@@ -35,6 +35,8 @@ module Gitlab
 
           metrics[:sidekiq_concurrency].set({}, Sidekiq.options[:concurrency].to_i)
 
+          return unless ::Feature.enabled?(:sidekiq_job_completion_metric_initialize, default_enabled: :yaml)
+
           ::Gitlab::SidekiqConfig.current_worker_queue_mappings.each do |worker, queue|
             worker_class = worker.safe_constantize
 

spec/lib/gitlab/sidekiq_middleware/server_metrics_spec.rb

@@ -58,6 +58,28 @@ RSpec.describe Gitlab::SidekiqMiddleware::ServerMetrics do
 
           described_class.initialize_process_metrics
         end
+
+        context 'when the sidekiq_job_completion_metric_initialize feature flag is disabled' do
+          before do
+            stub_feature_flags(sidekiq_job_completion_metric_initialize: false)
+          end
+
+          it 'sets the concurrency metric' do
+            expect(concurrency_metric).to receive(:set).with({}, Sidekiq.options[:concurrency].to_i)
+
+            described_class.initialize_process_metrics
+          end
+
+          it 'does not initialize sidekiq_jobs_completion_seconds' do
+            allow(Gitlab::SidekiqConfig)
+              .to receive(:current_worker_queue_mappings)
+                    .and_return('MergeWorker' => 'merge', 'BuildFinishedWorker' => 'default')
+
+            expect(completion_seconds_metric).not_to receive(:get)
+
+            described_class.initialize_process_metrics
+          end
+        end
       end
 
       describe '#call' do