Fully namespace importer in migrations

db/fixtures/development/99_common_metrics.rb

 # frozen_string_literal: true
 
-Importers::CommonMetrics::Importer.new.execute
+::Gitlab::Importers::CommonMetrics::Importer.new.execute

db/fixtures/production/999_common_metrics.rb

 # frozen_string_literal: true
 
-Importers::CommonMetrics::Importer.new.execute
+::Gitlab::Importers::CommonMetrics::Importer.new.execute

db/migrate/20181006004100_import_common_metrics_nginx_vts.rb

@@ -4,7 +4,7 @@ class ImportCommonMetricsNginxVts < ActiveRecord::Migration[5.0]
   DOWNTIME = false
 
   def up
-    Importers::CommonMetrics::Importer.new.execute
+    ::Gitlab::Importers::CommonMetrics::Importer.new.execute
   end
 
   def down

db/migrate/20190326164045_import_common_metrics_knative.rb

@@ -6,7 +6,7 @@ class ImportCommonMetricsKnative < ActiveRecord::Migration[5.0]
   DOWNTIME = false
 
   def up
-    Importers::CommonMetrics::Importer.new.execute
+    ::Gitlab::Importers::CommonMetrics::Importer.new.execute
   end
 
   def down

db/migrate/20190408163745_prometheus_knative05_fix.rb

@@ -9,7 +9,7 @@ class PrometheusKnative05Fix < ActiveRecord::Migration[5.0]
   DOWNTIME = false
 
   def up
-    Importers::CommonMetrics::Importer.new.execute
+    ::Gitlab::Importers::CommonMetrics::Importer.new.execute
   end
 
   def down

doc/development/prometheus_metrics.md

@@ -36,7 +36,7 @@ class ImportCommonMetrics < ActiveRecord::Migration[4.2]
   DOWNTIME = false
 
   def up
-    Importers::CommonMetrics::Importer.new.execute
+    ::Gitlab::Importers::CommonMetrics::Importer.new.execute
   end
 
   def down