Count total amount of users for each access level

Preparations to fill the users_statistics table with the highest role
counts by counting the total amount of users for each access level
stored in the users_highest_roles table.

app/models/user_highest_role.rb

@@ -4,4 +4,6 @@ class UserHighestRole < ApplicationRecord
   belongs_to :user, optional: false
 
   validates :highest_access_level, allow_nil: true, inclusion: { in: Gitlab::Access.all_values }
+
+  scope :with_highest_access_level, -> (highest_access_level) { where(highest_access_level: highest_access_level) }
 end

app/models/users_statistics.rb

@@ -11,4 +11,22 @@ class UsersStatistics < ApplicationRecord
     :bots,
     :blocked
   ].freeze
+
+  private
+
+  def highest_role_stats
+    return unless Feature.enabled?(:users_statistics)
+
+    {
+      owner: batch_count_for_access_level(Gitlab::Access::OWNER),
+      maintainer: batch_count_for_access_level(Gitlab::Access::MAINTAINER),
+      developer: batch_count_for_access_level(Gitlab::Access::DEVELOPER),
+      reporter: batch_count_for_access_level(Gitlab::Access::REPORTER),
+      guest: batch_count_for_access_level(Gitlab::Access::GUEST)
+    }
+  end
+
+  def batch_count_for_access_level(access_level)
+    Gitlab::Database::BatchCount.batch_count(UserHighestRole.with_highest_access_level(access_level))
+  end
 end

ee/app/models/ee/project_authorization.rb

@@ -13,8 +13,8 @@ module EE
         pluck(:user_id)
       end
 
-      # Get amout of users with highest role they have.
-      # If John is developer in one project but maintainer in another he will be
+      # Get amount of users with highest role they have.
+      # If John is a developer in one project but maintainer in another he will be
       # counted once as maintainer. This is needed to count users who don't use
       # functionality available to higher roles only.
       #

spec/factories/user_highest_roles.rb

@@ -3,5 +3,13 @@
 FactoryBot.define do
   factory :user_highest_role do
     user
+
+    trait :maintainer do
+      highest_access_level { Gitlab::Access::MAINTAINER }
+    end
+
+    trait :developer do
+      highest_access_level { Gitlab::Access::DEVELOPER }
+    end
   end
 end

spec/models/user_highest_role_spec.rb

@@ -10,4 +10,20 @@ describe UserHighestRole do
   describe 'validations' do
     it { is_expected.to validate_inclusion_of(:highest_access_level).in_array([nil, *Gitlab::Access.all_values]) }
   end
+
+  describe 'scopes' do
+    describe '.with_highest_access_level' do
+      let(:developer_access_level) { Gitlab::Access::DEVELOPER }
+      let!(:developer) { create(:user_highest_role, :developer) }
+      let!(:another_developer) { create(:user_highest_role, :developer) }
+      let!(:maintainer) { create(:user_highest_role, :maintainer) }
+
+      it 'only returns entry for developer access level' do
+        expect(described_class.with_highest_access_level(developer_access_level)).to contain_exactly(
+          developer,
+          another_developer
+        )
+      end
+    end
+  end
 end