Add DatabaseMetric specs

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

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::Usage::Metrics::Instrumentations::DatabaseMetric do
+  subject do
+    described_class.tap do |m|
+      m.relation { Issue }
+      m.operation :count
+      m.start { m.relation.minimum(:id) }
+      m.finish { m.relation.maximum(:id) }
+    end.new(time_frame: 'all')
+  end
+
+  describe '#value' do
+    let_it_be(:issue_1) { create(:issue, id: 314) }
+    let_it_be(:issue_2) { create(:issue, id: 451) }
+    let_it_be(:issue_3) { create(:issue, id: 949) }
+
+    before do
+      allow(ActiveRecord::Base.connection).to receive(:transaction_open?).and_return(false)
+    end
+
+    it 'calculates a correct result' do
+      expect(subject.value).to eq(3)
+    end
+
+    it 'caches the result of start and finish', :use_clean_rails_redis_caching do
+      subject.value
+
+      expect(Rails.cache.read('metric_instrumentation/issues_minimum_id')).to eq(314)
+      expect(Rails.cache.read('metric_instrumentation/issues_maximum_id')).to eq(949)
+    end
+  end
+end