Start UnicornSampler in master process

Using `on_master_start` assures that the sampler is started in
master process and not in worker processes.

changelogs/unreleased/unicorn-sampler-fix.yml

+---
+title: Make sure UnicornSampler is started only in master process.
+merge_request: 30215
+author:
+type: fixed

config/initializers/7_prometheus_metrics.rb

@@ -29,15 +29,13 @@ if !Rails.env.test? && Gitlab::Metrics.prometheus_metrics_enabled?
   Gitlab::Cluster::LifecycleEvents.on_worker_start do
     defined?(::Prometheus::Client.reinitialize_on_pid_change) && Prometheus::Client.reinitialize_on_pid_change
 
-    if defined?(::Unicorn)
-      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
 
-  if defined?(::Puma)
   Gitlab::Cluster::LifecycleEvents.on_master_start do
+    if defined?(::Unicorn)
+      Gitlab::Metrics::Samplers::UnicornSampler.initialize_instance(Settings.monitoring.unicorn_sampler_interval).start
+    elsif defined?(::Puma)
       Gitlab::Metrics::Samplers::PumaSampler.initialize_instance(Settings.monitoring.puma_sampler_interval).start
     end
   end