Motivation:

We already apply `HttpMessageDiscardWatchdogClientFilter` to
single-address clients to detect response leaks that could be caused by
unhandled exceptions in the filter chain. Multi-address client adds more
logic around a client group and therefore also could leak responses if
case on unexpected exceptions.

Modifications:

- Apply `HttpMessageDiscardWatchdogClientFilter` to multi-address client
instances;
- Enhance `HttpMessageDiscardWatchdogClientFilter` to include unhandled
exception in the logs to help narrow down the leak cause;

Result:

Users will see warn message if their multi-address client leaks
responses.