Disable the background Prometheus sampler in tests

This sampler can interfere with tests and cause transient build failures,
such as https://gitlab.com/gitlab-org/gitlab-ce/-/jobs/70241683.

config/initializers/7_prometheus_metrics.rb

@@ -25,7 +25,7 @@ Sidekiq.configure_server do |config|
   end
 end
 
-if Gitlab::Metrics.prometheus_metrics_enabled?
+if !Rails.env.test? && Gitlab::Metrics.prometheus_metrics_enabled?
   unless Sidekiq.server?
     Gitlab::Metrics::Samplers::UnicornSampler.initialize_instance(Settings.monitoring.unicorn_sampler_interval).start
   end