Correctly handle temporary folder for testing multiproces metrics

Gemfile

@@ -269,9 +269,9 @@ group :metrics do
   gem 'method_source', '~> 0.8', require: false
   gem 'influxdb', '~> 0.2', require: false
-# Prometheus
+  # Prometheus
   gem 'mmap2', '~> 2.2.6'
-  gem 'prometheus-client-mmap'
+  gem 'prometheus-client-mmap', '~>0.7.0.beta3'
 end
 group :development do

Gemfile.lock

@@ -1000,7 +1000,7 @@ DEPENDENCIES
   pg (~> 0.18.2)
   poltergeist (~> 1.9.0)
   premailer-rails (~> 1.9.0)
-  prometheus-client-mmap
+  prometheus-client-mmap (~> 0.7.0.beta3)
   pry-byebug (~> 3.4.1)
   pry-rails (~> 0.3.4)
   rack-attack (~> 4.4.1)

app/controllers/metrics_controller.rb

@@ -7,7 +7,7 @@ class MetricsController < ActionController::Base
   CHECKS = [
     Gitlab::HealthChecks::DbCheck,
     Gitlab::HealthChecks::RedisCheck,
-    Gitlab::HealthChecks::FsShardsCheck,
+    Gitlab::HealthChecks::FsShardsCheck
   ].freeze
   def metrics

spec/controllers/metrics_controller_spec.rb

@@ -6,8 +6,17 @@ describe MetricsController do
   let(:token) { current_application_settings.health_check_access_token }
   let(:json_response) { JSON.parse(response.body) }
+  around do |examples|
+    Dir.mktmpdir do |tmp_dir|
+      @metrics_multiproc_dir = tmp_dir
+      examples.run
+    end
+  end
+
   before do
     stub_env('IN_MEMORY_APPLICATION_SETTINGS', 'false')
+    stub_env('prometheus_multiproc_dir', @metrics_multiproc_dir)
+    allow(Gitlab::Metrics).to receive(:prometheus_metrics_enabled?).and_return(true)
   end
   describe '#metrics' do

spec/lib/gitlab/metrics_spec.rb

@@ -243,7 +243,6 @@ describe Gitlab::Metrics do
       subject { described_class.counter(:counter, 'doc') }
       it { is_expected.to be_a(Gitlab::Metrics::DummyMetric) }
-
     end
     describe '#summary' do