Track historical space stats in PgHero to determine PostgreSQL growth (#8906)

db/migrate/20181007025445_create_pghero_space_stats.rb

@@ -0,0 +1,13 @@
+class CreatePgheroSpaceStats < ActiveRecord::Migration[5.2]
+  def change
+    create_table :pghero_space_stats do |t|
+      t.text :database
+      t.text :schema
+      t.text :relation
+      t.integer :size, limit: 8
+      t.timestamp :captured_at
+    end
+
+    add_index :pghero_space_stats, [:database, :captured_at]
+  end
+end