Merge remote-tracking branch 'gitlab-ce/5029-support-cluster-metrics-ce' into 5029-support-cluster-metrics

# Conflicts:
#	app/controllers/projects/prometheus/metrics_controller.rb
#	app/controllers/projects/prometheus_controller.rb
#	app/models/environment.rb
#	config/gitlab.yml.example
#	config/sidekiq_queues.yml
#	db/schema.rb
#	spec/controllers/projects/prometheus/metrics_controller_spec.rb
#	spec/models/environment_spec.rb
#	spec/models/project_services/prometheus_service_spec.rb