Fix GrpcLoggingServiceTest hang on client error (#38082)

Author: aIbrahiim

runners/java-fn-execution/src/test/java/org/apache/beam/runners/fnexecution/logging/GrpcLoggingServiceTest.java

@@ -117,6 +117,7 @@ public void testMultipleClientsFailingIsHandledGracefullyByServer() throws Excep
             BeamFnLoggingGrpc.newStub(channel)
                 .logging(
                     TestStreams.withOnNext(messageDiscarder)
+                        .withOnCompleted(new CountDown(waitForTermination))
                         .withOnError(new CountDown(waitForTermination))
                         .build());
         outboundObserver.onNext(createLogsWithIds(instructionId, -instructionId));