protected String invokeToString() {
        StringBuilder sb = new StringBuilder();
        sb.append(iface.getName());
        sb.append(getSerializableFields());
        return sb.toString();
    }
Returns a Multimap from fieldName Strings to their getter Methods.

It expects to be called only from TaskSerDe. Multimap is used inside org.embulk.config.
embulk-deps/src/main/java/org/embulk/deps/config/TaskInvocationHandler.java