ActionController::UnknownFormat aka whitepage in dashboard/milestones

https://sentry.gitlap.com/gitlab/gitlabcom/issues/3103/

ActionController::UnknownFormat: ActionController::UnknownFormat
  app/controllers/dashboard_controller.rb:39:in `milestones'
    respond_to do |format|
  lib/gitlab/metrics/rack_middleware.rb:17:in `block in call'
    retval = trans.run { @app.call(env) }
  lib/gitlab/metrics/transaction.rb:45:in `run'
    yield
  lib/gitlab/metrics/rack_middleware.rb:17:in `call'
    retval = trans.run { @app.call(env) }
  lib/gitlab/middleware/go.rb:16:in `call'
    @app.call(env)
...
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