Set the Etag cache polling interval

app/controllers/projects/pipelines_controller.rb

@@ -29,6 +29,8 @@ class Projects::PipelinesController < Projects::ApplicationController
     respond_to do |format|
       format.html
       format.json do
+        Gitlab::PollingInterval.set_header(response, interval: 10_000)
+
         render json: {
           pipelines: PipelineSerializer
             .new(project: @project, user: @current_user)