Add OrphanedNamespacesMetric

config/metrics/counts_all/20241014113429_orphaned_namespaces.yml

+---
+key_path: counts.orphaned_namespaces
+description: Whether orphaned namespaces are present
+product_group: tenant_scale
+value_type: number
+status: active
+milestone: "17.6"
+instrumentation_class: OrphanedNamespacesMetric
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/168983
+time_frame: all
+data_source: database
+data_category: optional
+distribution:
+- ce
+- ee
+tier:
+- free
+- premium
+- ultimate

lib/gitlab/usage/metrics/instrumentations/orphaned_namespaces_metric.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module Usage
+    module Metrics
+      module Instrumentations
+        class OrphanedNamespacesMetric < GenericMetric
+          value do
+            ::Namespace
+              .where.not(parent_id: nil)
+              .where('NOT EXISTS(SELECT 1 FROM namespaces p WHERE p.id = namespaces.parent_id)')
+              .exists?
+          end
+        end
+      end
+    end
+  end
+end

spec/lib/gitlab/usage/metrics/instrumentations/orphaned_namespaces_metric_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::Usage::Metrics::Instrumentations::OrphanedNamespacesMetric, feature_category: :service_ping do
+  let(:expected_value) { false }
+
+  it_behaves_like 'a correct instrumented metric value', { time_frame: 'all', data_source: 'database' }
+end