Add tables to support geo secondary usage data

- Add new table to geo secondary that stores attributes as
  json

Relates to https://gitlab.com/gitlab-org/gitlab/issues/298781

ee/app/models/geo/secondary_usage_data.rb

+# frozen_string_literal: true
+
+# This class is used to store usage data on a secondary for transmission
+# to the primary during a status update.
+class Geo::SecondaryUsageData < Geo::TrackingBase
+  # Eventually, we'll find a way to auto-load this
+  # from the metric yaml files that include something
+  # like `run_on_secondary: true`, but for now we'll
+  # just enumerate them.
+  PAYLOAD_COUNT_FIELDS = %w(
+    git_fetch_event_count
+  ).freeze
+
+  store_accessor :payload, *PAYLOAD_COUNT_FIELDS
+  validate :payload_schema_is_valid
+
+  def payload_schema_is_valid
+    payload.keys.each do |key|
+      if PAYLOAD_COUNT_FIELDS.include?(key)
+        errors.add(:payload, "payload[#{key}] must be a number") unless payload[key].nil? || payload[key].is_a?(Numeric)
+      else
+        errors.add(:payload, "unexpected key in payload - #{key}")
+      end
+    end
+  end
+end

ee/changelogs/unreleased/298781-add_table_for_geo_usage_stats.yml

+---
+title: Add table to store secondary usage data
+merge_request: 53758
+author:
+type: added

ee/db/geo/migrate/20210208175408_create_geo_secondary_usage_data.rb

+# frozen_string_literal: true
+
+class CreateGeoSecondaryUsageData < ActiveRecord::Migration[6.0]
+  DOWNTIME = false
+
+  def change
+    create_table :secondary_usage_data do |t|
+      t.timestamps_with_timezone
+      t.jsonb :payload, null: false, default: {}
+    end
+  end
+end

ee/db/geo/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2020_12_08_031224) do
+ActiveRecord::Schema.define(version: 2021_02_08_175408) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -186,6 +186,12 @@ ActiveRecord::Schema.define(version: 2020_12_08_031224) do
     t.index ["wiki_verification_checksum_sha"], name: "idx_project_registry_on_wiki_checksum_sha_partial", where: "(wiki_verification_checksum_sha IS NULL)"
   end
 
+  create_table "secondary_usage_data", force: :cascade do |t|
+    t.datetime_with_timezone "created_at", null: false
+    t.datetime_with_timezone "updated_at", null: false
+    t.jsonb "payload", default: {}, null: false
+  end
+
   create_table "snippet_repository_registry", force: :cascade do |t|
     t.datetime_with_timezone "retry_at"
     t.datetime_with_timezone "last_synced_at"

ee/spec/factories/geo/secondary_usage_data.rb

+# frozen_string_literal: true
+
+FactoryBot.define do
+  factory :geo_secondary_usage_data, class: 'Geo::SecondaryUsageData' do
+    Geo::SecondaryUsageData::PAYLOAD_COUNT_FIELDS.each do |field|
+      send(field) { rand(10000) }
+    end
+  end
+end

ee/spec/models/geo/secondary_usage_data_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Geo::SecondaryUsageData, :geo, type: :model do
+  subject { create(:geo_secondary_usage_data) }
+
+  it 'is valid' do
+    expect(subject).to be_valid
+  end
+
+  it 'cannot have undefined fields in the payload' do
+    subject.payload['nope_does_not_exist'] = 'whatever'
+    expect(subject).not_to be_valid
+  end
+
+  shared_examples_for 'a payload count field' do |field|
+    it "defines #{field} as a method" do
+      expect(subject.methods).to include(field.to_sym)
+    end
+
+    it "does not allow #{field} to be a string" do
+      subject.payload[field] = 'a string'
+      expect(subject).not_to be_valid
+    end
+
+    it "allows #{field} to be nil" do
+      subject.payload[field] = nil
+      expect(subject).to be_valid
+    end
+
+    it "may not define #{field} in the payload json" do
+      subject.payload.except!(field)
+      expect(subject).to be_valid
+    end
+  end
+
+  Geo::SecondaryUsageData::PAYLOAD_COUNT_FIELDS.each do |field|
+    context "##{field}" do
+      it_behaves_like 'a payload count field', field
+    end
+  end
+end