private int flushCount() {
        return (int) MockitoUtils.countInvocations(offsetWriter, "doFlush", Callback.class);
    }
@return how many times an {@link OffsetStorageWriter#doFlush(Callback) offset flush} took place
connect/runtime/src/test/java/org/apache/kafka/connect/runtime/ExactlyOnceWorkerSourceTaskTest.java