Collect Sidekiq dead jobs count

Gemfile.lock

 PATH
   remote: .
   specs:
-    gitlab-monitor (1.4.0)
+    gitlab-monitor (1.5.0)
       pg (~> 0.18.4)
       quantile (~> 0.2.0)
       redis-namespace (~> 1.5.2)

config/gitlab-monitor.yml.example

@@ -76,5 +76,6 @@ probes:
       - probe_jobs
       - probe_workers
       - probe_retries
+      - probe_dead
     opts:
       redis_url: "redis://localhost:6379"

lib/gitlab_monitor/sidekiq.rb

@@ -95,6 +95,14 @@ module GitLab
         self
       end
 
+      def probe_dead
+        return self unless connected?
+
+        @metrics.add("sidekiq_dead_jobs_total", Sidekiq::Stats.new.dead_size)
+
+        self
+      end
+
       def write_to(target)
         target.write(@metrics.to_s)
       end

lib/gitlab_monitor/version.rb

 module GitLab
   module Monitor
-    VERSION = "1.4.0".freeze
+    VERSION = "1.5.0".freeze
   end
 end