Outlier detection for connections

[ikhoon]

Currently, there is no way to actively detect errors for specific connections and terminate connections that are unhealthy.

It would be useful when a server uses a load balancer as its endpoint. A host-level circuit breaker is not effective when a specific server behind the LB is not operating. If there are two servers and one server does not operate properly, the circuit may become open.

We may provide a OutlierDectionRule, similar to CircuitBreakerRule, to let users decide whether to close the connection if necessary. https://www.envoyproxy.io/docs/envoy/latest/intro/arch_overview/upstream/outlier