Add migration to import common metrics into DB

We recently changed the format for variable interpolation from
`%{}` to `{{}}`. The common_metrics.yml file has been updated. This
commit adds a migration to import the common_metrics.yml changes into
DB.

changelogs/unreleased/215563-migration-to-import-common-metrics.yml

+---
+title: Add migration to import changes to the system dashboard Prometheus queries
+  into DB
+merge_request: 31618
+author:
+type: changed

db/post_migrate/20200511145545_change_variable_interpolation_format_in_common_metrics.rb

+# frozen_string_literal: true
+
+class ChangeVariableInterpolationFormatInCommonMetrics < ActiveRecord::Migration[6.0]
+  DOWNTIME = false
+
+  def up
+    ::Gitlab::DatabaseImporters::CommonMetrics::Importer.new.execute
+  end
+
+  def down
+    # no-op
+    # The import cannot be reversed since we do not know the state that the
+    # common metrics in the PrometheusMetric table were in before the import.
+  end
+end

db/structure.sql

@@ -13765,5 +13765,6 @@ COPY "schema_migrations" (version) FROM STDIN;
 20200506085748
 20200506125731
 20200507221434
+20200511145545
 \.
 

spec/migrations/20200511145545_change_variable_interpolation_format_in_common_metrics_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+require Rails.root.join('db', 'post_migrate', '20200511145545_change_variable_interpolation_format_in_common_metrics')
+
+describe ChangeVariableInterpolationFormatInCommonMetrics, :migration do
+  let(:prometheus_metrics) { table(:prometheus_metrics) }
+
+  let!(:common_metric) do
+    prometheus_metrics.create!(
+      identifier: 'system_metrics_kubernetes_container_memory_total',
+      query: 'avg(sum(container_memory_usage_bytes{container_name!="POD",' \
+      'pod_name=~"^%{ci_environment_slug}-(.*)",namespace="%{kube_namespace}"})' \
+      ' by (job)) without (job)  /1024/1024/1024',
+      project_id: nil,
+      title: 'Memory Usage (Total)',
+      y_label: 'Total Memory Used (GB)',
+      unit: 'GB',
+      legend: 'Total (GB)',
+      group: -5,
+      common: true
+    )
+  end
+
+  it 'updates query to use {{}}' do
+    expected_query = 'avg(sum(container_memory_usage_bytes{container_name!="POD",' \
+    'pod_name=~"^{{ci_environment_slug}}-(.*)",namespace="{{kube_namespace}}"})' \
+    ' by (job)) without (job)  /1024/1024/1024'
+
+    migrate!
+
+    expect(common_metric.reload.query).to eq(expected_query)
+  end
+end