Don't export http_request_duration_seconds metrics in sidekiq exporter

The sidekiq exporter does not use `Gitlab::Metrics::RequestsRackMiddleware`
and `http_request_duration_seconds` bucket because it uses the WEBrick server
which does not use any middlewares at all.

Therefore, we don't need to initialize those metrics so the are not
exported which saves traffic and Prometheus storage.

changelogs/unreleased/250671-don-t-init-http-metrics-in-sidekiq.yml

+---
+title: Don't expose http_request_duration_seconds metrics in sidekiq exporter
+merge_request: 43941
+author:
+type: performance

config/initializers/7_prometheus_metrics.rb

@@ -69,7 +69,9 @@ if !Rails.env.test? && Gitlab::Metrics.prometheus_metrics_enabled?
 
     Gitlab::Metrics.gauge(:deployments, 'GitLab Version', {}, :max).set({ version: Gitlab::VERSION }, 1)
 
-    Gitlab::Metrics::RequestsRackMiddleware.initialize_http_request_duration_seconds
+    unless Gitlab::Runtime.sidekiq?
+      Gitlab::Metrics::RequestsRackMiddleware.initialize_http_request_duration_seconds
+    end
   rescue IOError => e
     Gitlab::ErrorTracking.track_exception(e)
     Gitlab::Metrics.error_detected!