Add external status checks total to ping

Adds the total number of external status checks
for an instance to the Service Ping

Changelog: added
EE: true

ee/config/metrics/counts_all/20220308155129_external_status_checks.yml

+---
+key_path: counts.external_status_checks
+description: Total number of external status checks
+product_section: dev
+product_stage: manage
+product_group: compliance
+product_category: "Compliance Management"
+value_type: number
+status: active
+milestone: "14.9"
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/82416
+time_frame: all
+data_source: database
+data_category: optional
+instrumentation_class: CountExternalStatusChecksMetric
+distribution:
+- ee
+tier:
+- ultimate

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

+# frozen_string_literal: true
+
+module Gitlab
+  module Usage
+    module Metrics
+      module Instrumentations
+        class CountExternalStatusChecksMetric < DatabaseMetric
+          operation :count
+
+          relation do
+            ::MergeRequests::ExternalStatusCheck
+          end
+        end
+      end
+    end
+  end
+end

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

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::Usage::Metrics::Instrumentations::CountExternalStatusChecksMetric do
+  let_it_be(:checks) { create_list(:external_status_check, 3) }
+
+  it_behaves_like 'a correct instrumented metric value and query', { time_frame: 'all', data_source: 'database' } do
+    let(:expected_value) { 3 }
+    let(:expected_query) { "SELECT COUNT(\"external_status_checks\".\"id\") FROM \"external_status_checks\"" }
+  end
+end