Add usage ping count for events distinct author_id

As part of usage ping, I would want to know how many users are
creating events (https://docs.gitlab.com/ee/api/events.html)

ee/app/models/ee/event.rb

@@ -6,6 +6,8 @@ module EE
     extend ::Gitlab::Utils::Override
 
     prepended do
+      include UsageStatistics
+
       scope :issues, -> { where(target_type: 'Issue') }
       scope :merge_requests, -> { where(target_type: 'MergeRequest') }
       scope :created, -> { where(action: ::Event::CREATED) }

ee/changelogs/unreleased/199874-add-new-usage-ping-counter-for-events.yml

+---
+title: Add usage ping counter for events
+merge_request: 199874
+author:
+type: added

ee/lib/ee/gitlab/usage_data.rb

@@ -233,6 +233,7 @@ module EE
         # Omitted because no user, creator or author associated: `campaigns_imported_from_github`, `ldap_group_links`
         def usage_activity_by_stage_manage
           {
+            events: ::Event.distinct_count_by(:author_id),
             groups: ::GroupMember.distinct_count_by(:user_id),
             ldap_keys: ::LDAPKey.distinct_count_by(:user_id),
             ldap_users: ::GroupMember.of_ldap_type.distinct_count_by(:user_id)

ee/spec/lib/ee/gitlab/usage_data_spec.rb

@@ -111,11 +111,13 @@ describe Gitlab::UsageData do
       context 'for manage' do
         it 'includes accurate usage_activity_by_stage data' do
           user = create(:user)
+          create(:event, author: user)
           create(:group_member, user: user)
           create(:key, type: 'LDAPKey', user: user)
           create(:group_member, ldap: true, user: user)
 
           expect(described_class.uncached_data[:usage_activity_by_stage][:manage]).to eq(
+            events: 1,
             groups: 1,
             ldap_keys: 1,
             ldap_users: 1