Child circuit breakers rely on proper matching of acquire/release pairs.
This can be tricky to get right. If we get it wrong and accidentally
double-release a CB then it may end up with a negative `used` value.
This is definitely a bad situation in which to find ourselves, but today
in production it's made a whole lot worse because it causes exceptions
on every attempt to report a `CircuitBreakerStats` or to construct a
parent `CircuitBreakingException`.

This commit makes the message construction and stats serialization a
little more robust so that it's clearer what is going on in production.

Relates elastic#86059