WIP: Add CPU & Memory metrics to k8s cluster page - backend

What does this MR do?

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #38783 (moved)

app/controllers/projects/clusters_controller.rb

@@ -63,6 +63,21 @@ def destroy
     end
   end
 
+  def metrics
+    # return render_404 unless deployment.has_additional_metrics?
+    respond_to do |format|
+      format.json do
+        metrics = PrometheusQuerier.new(cluster.application_prometheus).cluster_metrics(cluster) || {}
+
+        if metrics.any?
+          render json: metrics
+        else
+          head :no_content
+        end
+      end
+    end
+  end
+
   private
 
   def cluster