Update prometheus-client-mmap gem to highly optimized version

+ change string concatenation to help with GC pressure.
+ fix metric producing incompatible label sets

Gemfile.lock

@@ -486,7 +486,6 @@ GEM
     mini_mime (0.1.4)
     mini_portile2 (2.3.0)
     minitest (5.7.0)
-    mmap2 (2.2.9)
     mousetrap-rails (1.4.6)
     multi_json (1.12.2)
     multi_xml (0.6.0)
@@ -623,8 +622,7 @@ GEM
       parser
       unparser
     procto (0.0.3)
-    prometheus-client-mmap (0.7.0.beta39)
-      mmap2 (~> 2.2, >= 2.2.9)
+    prometheus-client-mmap (0.7.0.beta43)
     pry (0.10.4)
       coderay (~> 1.1.0)
       method_source (~> 0.8.1)

app/services/metrics_service.rb

@@ -20,7 +20,7 @@ class MetricsService
   end
 
   def metrics_text
-    "#{health_metrics_text}#{prometheus_metrics_text}"
+    prometheus_metrics_text.concat(health_metrics_text)
   end
 
   private

config/initializers/7_prometheus_metrics.rb

@@ -11,14 +11,7 @@ Prometheus::Client.configure do |config|
     config.multiprocess_files_dir ||= Rails.root.join('tmp/prometheus_multiproc_dir')
   end
 
-  config.pid_provider = -> do
-    worker_id = Prometheus::Client::Support::Unicorn.worker_id
-    if worker_id.nil?
-      "process_pid_#{Process.pid}"
-    else
-      "worker_id_#{worker_id}"
-    end
-  end
+  config.pid_provider = Prometheus::Client::Support::Unicorn.method(:worker_pid_provider)
 end
 
 Gitlab::Application.configure do |config|

lib/gitlab/metrics/samplers/ruby_sampler.rb

@@ -32,7 +32,7 @@ module Gitlab
 
         def init_metrics
           metrics = {}
-          metrics[:sampler_duration] = Metrics.histogram(with_prefix(:sampler_duration, :seconds), 'Sampler time', {})
+          metrics[:sampler_duration] = Metrics.histogram(with_prefix(:sampler_duration, :seconds), 'Sampler time', { worker: nil })
           metrics[:total_time] = Metrics.gauge(with_prefix(:gc, :time_total), 'Total GC time', labels, :livesum)
           GC.stat.keys.each do |key|
             metrics[key] = Metrics.gauge(with_prefix(:gc, key), to_doc_string(key), labels, :livesum)
@@ -100,9 +100,9 @@ module Gitlab
           worker_no = ::Prometheus::Client::Support::Unicorn.worker_id
 
           if worker_no
-            { unicorn: worker_no }
+            { worker: worker_no }
           else
-            { unicorn: 'master' }
+            { worker: 'master' }
           end
         end
       end