[stats] Add support for `track_remaining` in Circuit Breaker configuration

[Jean-Daniel]

Description:
Envoy Circuit Breaker configuration supports the track_remaining boolean to collection of the number of resources remaining until the circuit breakers open in envoy and expose them in envoy stats. This field defaults to false and cannot be enabled when using Envoy Gateway.

When this field is true, some additional circuit breaker metrics are exposed (in prometheus, for instance)

Such metrics are extremely useful to proactively detect if a circuit breaker is about to open. Can this field be exposed in BackendTrafficPolicy alongside other circuit breaker settings ?

[Relevant Links:]
Envoy reference for track_remaining

Envoy Circuit Breaker stats documentation

[zirain]

cc @guydc