Add database.flavor to usage data

config/metrics/settings/20211201012652_flavor.yml

doc/development/service_ping/index.md

ee/spec/lib/ee/gitlab/usage_data_non_sql_metrics_spec.rb

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

lib/gitlab/usage_data.rb

...controllers/admin/application_settings_controller_spec.rb

spec/controllers/admin/instance_review_controller_spec.rb

spec/features/admin/admin_settings_spec.rb

spec/lib/gitlab/usage_data_spec.rb

spec/requests/api/usage_data_non_sql_metrics_spec.rb

spec/requests/api/usage_data_queries_spec.rb

...services/service_ping/submit_service_ping_service_spec.rb

spec/support/helpers/usage_data_helpers.rb

...ping_payload_with_all_expected_metrics_shared_examples.rb

...ing_payload_without_restricted_metrics_shared_examples.rb

spec/tasks/gitlab/usage_data_rake_spec.rb