Expose concurrent worker metrics to prom

What does this MR do and why?

corrective action from gitlab-com/gl-infra/production#18146 (closed) and addresses #437646

This MR adds a new sidekiq worker metric to prometheus.

I added it to the ConcurrencyLimit::ResumeWorker so it will only run for workers that have this middleware enabled. The ConcurrencyLimit::ResumeWorker runs every minute and will report the metric whether there are jobs in the queue or not.

MR acceptance checklist

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Screenshots or screen recordings

N/A

How to set up and validate locally

patch for testing

diff --git a/ee/app/workers/zoekt/indexer_worker.rb b/ee/app/workers/zoekt/indexer_worker.rb
index 70bbeacd7d16..8fc3b42713eb 100644
--- a/ee/app/workers/zoekt/indexer_worker.rb
+++ b/ee/app/workers/zoekt/indexer_worker.rb
@@ -20,9 +20,10 @@ class IndexerWorker
     sidekiq_options retry: 2
     idempotent!
     pause_control :zoekt
-    concurrency_limit -> { 30 if Feature.enabled?(:zoekt_limit_indexing_concurrency) }
+    concurrency_limit -> { 1 }
 
     def perform(project_id, options = {})
+      sleep(30)
       return unless ::Gitlab::CurrentSettings.zoekt_indexing_enabled?
       return unless ::License.feature_available?(:zoekt_code_search)
 

1. enable sidekiq metrics in gdk

 gdk config set gitlab.rails_background_jobs.sidekiq_exporter_enabled true
 gdk reconfigure

2. apply the patch above to manually set this worker very low, and add a sleep timer to cause the worker queue to back up
3. in rails console, queue the worker for every project in your database

    Project.all.each { |p| Zoekt::IndexerWorker.perform_async(p.id) }

4. load the metrics page for sidekiq metrics: http://gdk.test:3807/metrics
5. ensure the values are updated for the metric

# HELP sidekiq_concurrency_limit_max_concurrent_jobs Multiprocess metric
# TYPE sidekiq_concurrency_limit_max_concurrent_jobs gauge
sidekiq_concurrency_limit_max_concurrent_jobs{worker="ElasticCommitIndexerWorker",pid="sidekiq_0"} 30
sidekiq_concurrency_limit_max_concurrent_jobs{worker="PackageMetadata::GlobalAdvisoryScanWorker",pid="sidekiq_0"} 10
sidekiq_concurrency_limit_max_concurrent_jobs{worker="Search::Zoekt::DeleteProjectWorker",pid="sidekiq_0"} 100
sidekiq_concurrency_limit_max_concurrent_jobs{worker="Security::ScanExecutionPolicies::CreatePipelineWorker",pid="sidekiq_0"} 10000
sidekiq_concurrency_limit_max_concurrent_jobs{worker="Security::ScanResultPolicies::AddApproversToRulesWorker",pid="sidekiq_0"} 200
sidekiq_concurrency_limit_max_concurrent_jobs{worker="Zoekt::IndexerWorker",pid="sidekiq_0"} 1
# HELP sidekiq_concurrency_limit_queue_size Multiprocess metric
# TYPE sidekiq_concurrency_limit_queue_size gauge
sidekiq_concurrency_limit_queue_size{worker="ElasticCommitIndexerWorker"} 0
sidekiq_concurrency_limit_queue_size{worker="PackageMetadata::GlobalAdvisoryScanWorker"} 0
sidekiq_concurrency_limit_queue_size{worker="Search::Zoekt::DeleteProjectWorker"} 0
sidekiq_concurrency_limit_queue_size{worker="Security::ScanExecutionPolicies::CreatePipelineWorker"} 0
sidekiq_concurrency_limit_queue_size{worker="Security::ScanResultPolicies::AddApproversToRulesWorker"} 0
sidekiq_concurrency_limit_queue_size{worker="Zoekt::IndexerWorker"} 44