Add new metric tracking users that use deployment approvals

config/metrics/counts_28d/20220407183012_count_distinct_user_id_from_deployment_approvals.yml

+---
+key_path: usage_activity_by_stage_monthly.release.users_creating_deployment_approvals
+description: Users who have used the deployment approvals feature by month
+product_section: ops
+product_stage: release
+product_group: group::release
+product_category: continuous_delivery
+value_type: number
+status: active
+milestone: "14.10"
+introduced_by_url:
+time_frame: 28d
+data_source: database
+data_category: optional
+instrumentation_class: CountUsersDeploymentApprovals
+performance_indicator_type: []
+distribution:
+- ee
+tier:
+- premium
+- ultimate

config/metrics/counts_all/20220407183012_count_distinct_user_id_from_deployment_approvals.yml

+---
+key_path: usage_activity_by_stage.release.users_creating_deployment_approvals
+description: Users who have used the deployment approvals feature by all time
+product_section: ops
+product_stage: release
+product_group: group::release
+product_category: continuous_delivery
+value_type: number
+status: active
+milestone: "14.10"
+introduced_by_url:
+time_frame: all
+data_source: database
+data_category: optional
+instrumentation_class: CountUsersDeploymentApprovals
+performance_indicator_type: []
+distribution:
+- ee
+tier:
+- premium
+- ultimate

ee/lib/gitlab/usage/metrics/instrumentations/count_users_deployment_approvals.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module Usage
+    module Metrics
+      module Instrumentations
+        class CountUsersDeploymentApprovals < DatabaseMetric
+          operation :distinct_count, column: :user_id
+
+          relation { ::Deployments::Approval }
+        end
+      end
+    end
+  end
+end

ee/spec/lib/gitlab/usage/metrics/instrumentations/count_users_deployment_approvals_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::Usage::Metrics::Instrumentations::CountUsersDeploymentApprovals do
+  let_it_be(:user) { create(:user) }
+  let_it_be(:deployment_approval) { create(:deployment_approval, user: user) }
+  let_it_be(:deployment_approval2) { create(:deployment_approval, user: user) }
+
+  let(:expected_value) { 1 }
+  let(:expected_query) { 'SELECT COUNT(DISTINCT "deployment_approvals"."user_id") FROM "deployment_approvals"' }
+
+  it_behaves_like 'a correct instrumented metric value and query', { time_frame: 'all' }
+end