Overload manager

The overload manager is an extensible component for protecting the Envoy server from overload with respect to various system resources (such as memory, cpu or file descriptors) due to too many client connections or requests. This is distinct from circuit breaking which is primarily aimed at protecting upstream services.

The overload manager is configured by specifying a set of resources to monitor and a set of overload actions that will be taken when some of those resources exceed certain pressure thresholds.