Add cluster query spec

ee/spec/lib/gitlab/prometheus/queries/cluster_query_spec.rb

+require 'spec_helper'
+
+describe Gitlab::Prometheus::Queries::ClusterQuery do
+  let(:client) { double('prometheus_client', query_range: nil) }
+  subject { described_class.new(client) }
+
+  around do |example|
+    Timecop.freeze { example.run }
+  end
+
+  it 'load cluster metrics from yaml' do
+    expect(Gitlab::Prometheus::AdditionalMetricsParser).to receive(:load_groups_from_yaml).with('cluster_metrics.yml').and_call_original
+
+    subject.query
+  end
+
+  it 'sends queries to prometheus' do
+    subject.query
+
+    expect(client).to have_received(:query_range).with(anything, start: 8.hours.ago, stop: Time.now).at_least(1)
+  end
+end