circuit breaker: track remaining resource count (#6269)
circuit breaker: track remaining resource count (#6269)

This commit adds cluster stats tracking the resources remaining
until a circuit breaker opens.

Signed-off-by: Spencer Lewis <slewis@squareup.com>

Mirrored from https://github.com/envoyproxy/envoy @ 0cd4054747440e019a5acc62f33f0a3b5db4975f

lyft/envoy-api,lyft/envoy-api,lyft/envoy-api