Add a rules file for the node_exporter

* Include the upstream[0] example rules file.
* Trigger HUP of prometheus server.
* Update `node_exporter` spec.

[0]: https://github.com/prometheus/node_exporter/blob/master/example.rules

files/gitlab-cookbooks/gitlab/attributes/default.rb

@@ -895,6 +895,7 @@ default['gitlab']['prometheus']['target_heap_size'] = (
   # Use 25mb + 2% of total memory for Prometheus memory.
   26_214_400 + (node['memory']['total'].to_i * 1024 * 0.02)
 ).to_i
+default['gitlab']['prometheus']['rules_files'] = []
 ####
 # Prometheus Node Exporter

files/gitlab-cookbooks/gitlab/recipes/node-exporter.rb

@@ -16,6 +16,7 @@
 # limitations under the License.
 #
 account_helper = AccountHelper.new(node)
+prometheus_dir = node['gitlab']['prometheus']['home']
 prometheus_user = account_helper.prometheus_user
 node_exporter_log_dir = node['gitlab']['node-exporter']['log_directory']
 textfile_dir = File.join(node['gitlab']['node-exporter']['home'], 'textfile_collector')
@@ -52,3 +53,14 @@ if node['gitlab']['bootstrap']['enable']
     retries 20
   end
 end
+
+node_rules_file = File.join(prometheus_dir, 'node.rules')
+
+node.default['gitlab']['prometheus']['rules_files'] << node_rules_file
+
+template node_rules_file do
+  source 'prometheus/rules/node.rules.erb'
+  owner prometheus_user
+  mode '0644'
+  only_if { node['gitlab']['prometheus']['enable'] }
+end

files/gitlab-cookbooks/gitlab/templates/default/prometheus/rules/node.rules.erb

+# The count of CPUs per node, useful for getting CPU time as a percent of total.
+instance:node_cpus:count = count(node_cpu{mode="idle"}) without (cpu,mode)
+
+# CPU in use by CPU.
+instance_cpu:node_cpu_not_idle:rate5m = sum(rate(node_cpu{mode!="idle"}[5m])) without (mode)
+
+# CPU in use by mode.
+instance_mode:node_cpu:rate5m = sum(rate(node_cpu[5m])) without (cpu)

spec/chef/recipes/node-exporter_spec.rb

@@ -64,6 +64,20 @@ describe 'gitlab::node-exporter' do
     end
   end
+  context 'when node-exporter is enabled and prometheus is enabled' do
+    before do
+      stub_gitlab_rb(
+        prometheus: { enable: true },
+        node_exporter: { enable: true }
+      )
+    end
+
+    it 'creates a node.rules file' do
+      expect(chef_run).to render_file('/var/opt/gitlab/prometheus/node.rules')
+        .with_content(/instance:node_cpus:count/)
+    end
+  end
+
   context 'when log dir is changed' do
     before do
       stub_gitlab_rb(