Detect unexpected response leaks for multi-address client instances

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.

servicetalk-http-netty/src/main/java/io/servicetalk/http/netty/DefaultMultiAddressUrlHttpClientBuilder.java

@@ -135,6 +135,9 @@ public StreamingHttpClient buildStreaming() {
             urlClient = redirectConfig == null ? urlClient :
                     new RedirectingHttpRequesterFilter(redirectConfig).create(urlClient);
 
+            // Detect leaks that can be caused by unexpected exceptions
+            urlClient = HttpMessageDiscardWatchdogClientFilter.CLIENT_CLEANER.create(urlClient);
+
             LOGGER.debug("Multi-address client created with base strategy {}", executionContext.executionStrategy());
             return new FilterableClientToClient(urlClient, executionContext);
         } catch (final Throwable t) {

servicetalk-http-netty/src/main/java/io/servicetalk/http/netty/HttpMessageDiscardWatchdogClientFilter.java

@@ -115,7 +115,7 @@ protected Single<StreamingHttpResponse> request(final StreamingHttpRequester del
                                     LOGGER.warn("Discovered un-drained HTTP response message body which has " +
                                             "been dropped by user code - this is a strong indication of a bug " +
                                             "in a user-defined filter. Response payload (message) body must " +
-                                            "be fully consumed before discarding.");
+                                            "be fully consumed before discarding.", cause);
                                 }
                                 return Single.<StreamingHttpResponse>failed(cause).shareContextOnSubscribe();
                             });