Add service to generate user data export CSV

This service generates a CSV string which has current license info
in the file header followed by a table of historical user counts.

ee/app/services/historical_user_data/csv_service.rb

+# frozen_string_literal: true
+
+module HistoricalUserData
+  class CsvService
+    FILESIZE_LIMIT = 15.megabytes
+
+    def initialize(historical_data_relation)
+      @historical_data_relation = historical_data_relation
+    end
+
+    def generate
+      header_csv + csv_builder.render(FILESIZE_LIMIT)
+    end
+
+    private
+
+    attr_reader :historical_data_relation
+
+    def csv_builder
+      @csv_builder ||= CsvBuilder.new(historical_data_relation, header_to_value_hash)
+    end
+
+    def header_to_value_hash
+      {
+        'Date' => -> (historical_datum) { historical_datum.date.to_s(:csv) },
+        'Active User Count' => 'active_user_count'
+      }
+    end
+
+    def license
+      @license ||= License.current
+    end
+
+    def header_csv
+      CSV.generate do |csv|
+        csv << ['License Key', license.data]
+        csv << ['Email', license.licensee['Email']]
+        csv << ['License Start Date', license.starts_at.to_s(:csv)]
+        csv << ['License End Date', license.expires_at.to_s(:csv)]
+        csv << ['Company', license.licensee['Company']]
+        csv << ['Generated At', Time.current.to_s(:csv)]
+        csv << ['']
+      end
+    end
+  end
+end

ee/spec/services/historical_user_data/csv_service_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe HistoricalUserData::CsvService do
+  subject(:csv) { CSV.parse(csv_service.generate) }
+
+  let_it_be(:csv_service) { described_class.new(HistoricalData.all) }
+
+  let_it_be(:datetime) { DateTime.parse('2020-09-04 10:00:00') }
+  let_it_be(:license_email) { 'data@example.com' }
+  let_it_be(:license_company) { 'Widgets Inc.' }
+  let_it_be(:license_start_date) { datetime.to_date - 10.months }
+  let_it_be(:license_end_date) { datetime.to_date + 2.months }
+
+  let_it_be(:current_license) do
+    create_current_license(
+      licensee: { 'Name' => 'Test', 'Email' => license_email, 'Company' => license_company },
+      starts_at: license_start_date,
+      expires_at: license_end_date
+    )
+  end
+
+  around do |example|
+    travel_to(datetime) { example.run }
+  end
+
+  context 'License Information Header' do
+    context 'License Key' do
+      it 'shows the header title' do
+        expect(csv[0][0]).to eq('License Key')
+      end
+
+      it 'shows the license key' do
+        expect(csv[0][1]).to eq(current_license.data)
+      end
+    end
+
+    context 'Email' do
+      it 'shows the header title' do
+        expect(csv[1][0]).to eq('Email')
+      end
+
+      it 'shows the license email' do
+        expect(csv[1][1]).to eq(license_email)
+      end
+    end
+
+    context 'License Start Date' do
+      it 'shows the header title' do
+        expect(csv[2][0]).to eq('License Start Date')
+      end
+
+      it 'shows the license start date' do
+        expect(csv[2][1]).to eq(license_start_date.to_s(:csv))
+      end
+    end
+
+    context 'License End Date' do
+      it 'shows the header title' do
+        expect(csv[3][0]).to eq('License End Date')
+      end
+
+      it 'shows the license end date' do
+        expect(csv[3][1]).to eq(license_end_date.to_s(:csv))
+      end
+    end
+
+    context 'Company' do
+      it 'shows the header title' do
+        expect(csv[4][0]).to eq('Company')
+      end
+
+      it 'shows the license company' do
+        expect(csv[4][1]).to eq(license_company)
+      end
+    end
+
+    context 'Generated At' do
+      it 'shows the header title' do
+        expect(csv[5][0]).to eq('Generated At')
+      end
+
+      it 'shows the CSV generation time' do
+        expect(csv[5][1]).to eq(datetime.to_s(:csv))
+      end
+    end
+  end
+
+  context 'User Count Table' do
+    let_it_be(:historical_datum) do
+      create(:historical_data, date: license_start_date, active_user_count: 1)
+    end
+    let_it_be(:historical_datum2) do
+      create(:historical_data, date: license_start_date + 1.day, active_user_count: 2)
+    end
+
+    it 'shows the header for the user counts table' do
+      expect(csv[7]).to contain_exactly('Date', 'Active User Count')
+    end
+
+    it 'includes proper values for each column type', :aggregate_failures do
+      expect(csv[8]).to contain_exactly(
+        historical_datum.date.to_s(:db),
+        historical_datum.active_user_count.to_s
+      )
+      expect(csv[9]).to contain_exactly(
+        historical_datum2.date.to_s(:db),
+        historical_datum2.active_user_count.to_s
+      )
+    end
+  end
+end