Add migration to import common metrics

Add a migration to import the new common metrics queries after they
were updated to work with the pod/container labels as well as the older
pod_name/container_name labels.

changelogs/unreleased/rpereira2-add-migration-to-import-common-metrics.yml

+---
+title: Import the new queries in common_metrics.yml into database
+merge_request: 39475
+author:
+type: changed

db/post_migrate/20200813153434_import_latest_common_metrics.rb

+# frozen_string_literal: true
+
+class ImportLatestCommonMetrics < ActiveRecord::Migration[6.0]
+  DOWNTIME = false
+
+  def up
+    # The common_metrics queries were updated to work with K8s versions that
+    # use the pod/container label names as well as K8s versions that use the
+    # older pod_name/container_name convention.
+    ::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.
+
+    # To manually revert this migration.
+    # 1. Go back to the previous version of the config/prometheus/common_metrics.yml file. (git checkout 74447f11349617ed8b273196d6a5781d9a67a613)
+    # 2. Execute `rails runner '::Gitlab::DatabaseImporters::CommonMetrics::Importer.new.execute'`
+  end
+end

db/schema_migrations/20200813153434

+23de9421726337f5279bf434542bffda24cbadc7324b498ff642ec4c0883b5d5
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