Migrate add_user_count metric to use instrumentation class

config/metrics/license/20210204124829_active_user_count.yml

@@ -10,6 +10,7 @@ status: active
 milestone: "<13.9"
 data_category: subscription
 time_frame: none
+instrumentation_class: ActiveUserCountMetric
 data_source: database
 distribution:
 - ce

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

+# frozen_string_literal: true
+
+module Gitlab
+  module Usage
+    module Metrics
+      module Instrumentations
+        class ActiveUserCountMetric < DatabaseMetric
+          operation :count
+
+          relation { User.active }
+        end
+      end
+    end
+  end
+end

lib/gitlab/usage_data.rb

@@ -76,7 +76,7 @@ module Gitlab
           hostname: add_metric('HostnameMetric'),
           version: alt_usage_data { Gitlab::VERSION },
           installation_type: alt_usage_data { installation_type },
-          active_user_count: count(User.active),
+          active_user_count: add_metric('ActiveUserCountMetric'),
           edition: 'CE'
         }
       end

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

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::Usage::Metrics::Instrumentations::ActiveUserCountMetric do
+  before do
+    create(:user)
+  end
+
+  it_behaves_like 'a correct instrumented metric value', { time_frame: 'none', data_source: 'ruby' } do
+    let(:expected_value) { ::User.active.count }
+  end
+end