Improve MetricGroup specs

spec/lib/gitlab/prometheus/metric_group_spec.rb

@@ -12,7 +12,8 @@ describe Gitlab::Prometheus::MetricGroup do
     subject { described_class.common_metrics }
 
     it 'returns exactly two groups' do
-      expect(subject.map(&:name)).to contain_exactly('Response metrics (AWS ELB)', 'System metrics (Kubernetes)')
+      expect(subject.map(&:name)).to contain_exactly(
+        'Response metrics (AWS ELB)', 'System metrics (Kubernetes)')
     end
 
     it 'returns exactly three metric queries' do
@@ -27,10 +28,14 @@ describe Gitlab::Prometheus::MetricGroup do
     let!(:project_metric) { create(:prometheus_metric) }
     let!(:common_metric) { create(:prometheus_metric, :common, group: :aws_elb) }
 
-    subject { described_class.for_project(other_project) }
+    subject do
+      described_class.for_project(other_project)
+        .map(&:metrics).flatten
+        .map(&:id)
+    end
 
     it 'returns exactly one common metric' do
-      expect(subject.map(&:metrics).flatten.map(&:id)).to contain_exactly(common_metric.id)
+      is_expected.to contain_exactly(common_metric.id)
     end
   end
 end