remove common Base Sampler code

doc/user/project/integrations/prometheus.md

@@ -14,8 +14,6 @@ the settings page with a default template. To configure the template, see the
 
 ## Requirements
 
-[supported performance metrics](https://docs.gitlab.com/ee/user/project/integrations/prometheus_library/metrics.html)
-
 Integration with Prometheus requires the following:
 
 1. GitLab 9.0 or higher

lib/gitlab/metrics/samplers/base_sampler.rb

@@ -2,20 +2,7 @@ require 'logger'
 module Gitlab
   module Metrics
     module Samplers
-      class BaseSampler
-        def self.initialize_instance(*args)
-          raise "#{name} singleton instance already initialized" if @instance
-          @instance = new(*args)
-          at_exit(&@instance.method(:stop))
-          @instance
-        end
-
-        def self.instance
-          @instance
-        end
-
-        attr_reader :running
-
+      class BaseSampler < Daemon
         # interval - The sampling interval in seconds.
         def initialize(interval)
           interval_half = interval.to_f / 2
@@ -23,44 +10,7 @@ module Gitlab
           @interval = interval
           @interval_steps = (-interval_half..interval_half).step(0.1).to_a
 
-          @mutex = Mutex.new
-        end
-
-        def enabled?
-          true
-        end
-
-        def start
-          return unless enabled?
-
-          @mutex.synchronize do
-            return if running
-            @running = true
-
-            @thread = Thread.new do
-              sleep(sleep_interval)
-
-              while running
-                safe_sample
-
-                sleep(sleep_interval)
-              end
-            end
-          end
-        end
-
-        def stop
-          @mutex.synchronize do
-            return unless running
-
-            @running = false
-
-            if @thread
-              @thread.wakeup if @thread.alive?
-              @thread.join
-              @thread = nil
-            end
-          end
+          super()
         end
 
         def safe_sample
@@ -90,7 +40,22 @@ module Gitlab
             end
           end
         end
+
+        private
+
+        def start_working
+          @running = true
+          sleep(sleep_interval)
+          while running
+            safe_sample
+          end
+        end
+
+        def stop_working
+          @running = false
+        end
       end
     end
   end
 end
+