interop-testing: fix peer extraction issue in soak test iterations

This PR resolves an issue with peer address extraction in the soak
test. 

In current `TestServiceClient` implementation, the same
`clientCallCapture` atomic is shared across threads, leading to
incorrect peer extraction. This fix ensures that each thread uses a
local variable for capturing the client call.

Author: zbilun

interop-testing/src/main/java/io/grpc/testing/integration/AbstractInteropTest.java

@@ -1874,14 +1874,15 @@ private void executeSoakTestInThread(
       }
       long earliestNextStartNs = System.nanoTime()
           + TimeUnit.MILLISECONDS.toNanos(minTimeMsBetweenRpcs);
-
+      // recordClientCallInterceptor takes an AtomicReference.
+      AtomicReference<ClientCall<?, ?>> soakThreadClientCallCapture = new AtomicReference<>();
       currentChannel = maybeCreateChannel.apply(currentChannel);
       TestServiceGrpc.TestServiceBlockingStub currentStub = TestServiceGrpc
           .newBlockingStub(currentChannel)
-              .withInterceptors(recordClientCallInterceptor(clientCallCapture));
+              .withInterceptors(recordClientCallInterceptor(soakThreadClientCallCapture));
       SoakIterationResult result = performOneSoakIteration(currentStub,
           soakRequestSize, soakResponseSize);
-      SocketAddress peer = clientCallCapture
+      SocketAddress peer = soakThreadClientCallCapture
           .get().getAttributes().get(Grpc.TRANSPORT_ATTR_REMOTE_ADDR);
       StringBuilder logStr = new StringBuilder(
           String.format(