Add Prometheus metrics configuration

+ Cleanup Gemfile

Gemfile

@@ -268,6 +268,10 @@ group :metrics do
   gem 'allocations', '~> 1.0', require: false, platform: :mri
   gem 'method_source', '~> 0.8', require: false
   gem 'influxdb', '~> 0.2', require: false
+
+# Prometheus
+  gem 'mmap2', '~> 2.2.6'
+  gem 'prometheus-client-mmap'
 end
 
 group :development do

app/views/admin/application_settings/_form.html.haml

@@ -232,9 +232,9 @@
           = f.number_field :container_registry_token_expire_delay, class: 'form-control'
 
   %fieldset
-    %legend Metrics
+    %legend Metrics - Influx
     %p
-      Setup InfluxDB to measure a wide variety of statistics like the time spent
+      Setup Influx to measure a wide variety of statistics like the time spent
       in running SQL queries. These settings require a
       = link_to 'restart', help_page_path('administration/restart_gitlab')
       to take effect.
@@ -296,6 +296,21 @@
           The amount of points to store in a single UDP packet. More points
           results in fewer but larger UDP packets being sent.
 
+  %fieldset
+    %legend Metrics - Prometheus
+    %p
+      Setup Prometheus to measure a variety of statistics that partially overlap and complement Influx based metrics.
+      This setting requires a
+      = link_to 'restart', help_page_path('administration/restart_gitlab')
+      to take effect.
+      = link_to icon('question-circle'), help_page_path('administration/monitoring/performance/introduction')
+    .form-group
+      .col-sm-offset-2.col-sm-10
+        .checkbox
+          = f.label :prometheus_metrics_enabled do
+            = f.check_box :prometheus_metrics_enabled
+            Enable Prometheus Metrics
+
   %fieldset
     %legend Background Jobs
     %p

db/migrate/20170519102115_add_prometheus_settings_to_metrics_settings.rb

@@ -0,0 +1,13 @@
+class AddPrometheusSettingsToMetricsSettings < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+  disable_ddl_transaction!
+
+  def up
+    add_column_with_default(:application_settings, :prometheus_metrics_enabled, :boolean,
+                            default: false, allow_null: false)
+  end
+
+  def down
+    remove_column(:application_settings, :prometheus_metrics_enabled)
+  end
+end