Overload Manager should monitor epoll loop duration and polling delay as resources

The Overload Manager has a configurable set of resource monitors that can be used to trigger actions when Envoy is overloaded. Monitoring the epoll loop duration and polling delay would allow proxies to take action when they are over-taxed in terms of CPU, possibly with a large number of pending events to handle. This could be done by polling the epoll stat metric, but I'm not sure that's the most efficient option.