Maintain database parity between experiment_users & experiment_subjects

- Add converted_at(timestamp) column to experiment_subjects
- Add context(jsonb) column to experiment_subjects
- Add a changelog entry for database changes

changelogs/unreleased/297664-maintain-data-parity-between-experimentuser-experimentsubject.yml

+---
+title: Add converted_at(timestamp) & context(jsonb) columns to the experiment_subjects
+  table
+merge_request: 51659
+author:
+type: changed

db/migrate/20210113231532_add_converted_at_to_experiment_subjects.rb

+# frozen_string_literal: true
+
+class AddConvertedAtToExperimentSubjects < ActiveRecord::Migration[6.0]
+  DOWNTIME = false
+
+  def change
+    add_column :experiment_subjects, :converted_at, :datetime_with_timezone
+  end
+end

db/migrate/20210113231546_add_context_to_experiment_subjects.rb

+# frozen_string_literal: true
+
+class AddContextToExperimentSubjects < ActiveRecord::Migration[6.0]
+  DOWNTIME = false
+
+  def change
+    add_column :experiment_subjects, :context, :jsonb, default: {}, null: false
+  end
+end

db/schema_migrations/20210113231532

+1a430ce6d137ee896cbd37bb822ccd18c9e87204b765b1192928dd82efb28602
\ No newline at end of file

db/schema_migrations/20210113231546

+5d7bbf376acbf5679d111866e70b69eebba26a487a9e7d6bd571f15dc423e3e3
\ No newline at end of file

db/structure.sql

@@ -12261,6 +12261,8 @@ CREATE TABLE experiment_subjects (
     variant smallint DEFAULT 0 NOT NULL,
     created_at timestamp with time zone NOT NULL,
     updated_at timestamp with time zone NOT NULL,
+    converted_at timestamp with time zone,
+    context jsonb DEFAULT '{}'::jsonb NOT NULL,
     CONSTRAINT chk_has_one_subject CHECK ((num_nonnulls(user_id, group_id, project_id) = 1))
 );
 

spec/db/schema_spec.rb

@@ -184,6 +184,7 @@ RSpec.describe 'Database schema' do
     "ApplicationSetting" => %w[repository_storages_weighted],
     "AlertManagement::Alert" => %w[payload],
     "Ci::BuildMetadata" => %w[config_options config_variables],
+    "ExperimentSubject" => %w[context],
     "ExperimentUser" => %w[context],
     "Geo::Event" => %w[payload],
     "GeoNodeStatus" => %w[status],