How to use the httpstan.callbacks_writer_pb2.WriterMessage.Topic function in httpstan

# These names are gathered later in this function by inspecting the output from Stan.
        self.sample_and_sampler_param_names: Sequence[str]

        num_flat_params = sum(np.product(dims_ or 1) for dims_ in dims)  # if dims == [] then it is a scalar
        assert num_flat_params == len(constrained_param_names)
        num_samples_saved = (self.num_samples + self.num_warmup * self.save_warmup) // self.num_thin

        # self._draws holds all the draws. We cannot allocate it before looking at the draws
        # because we do not know how many sampler-specific parameters are present. Later in this
        # function we count them and only then allocate the array for `self._draws`.
        self._draws: np.ndarray

        for chain_index, stan_output in zip(range(self.num_chains), self.stan_outputs):
            draw_index = 0
            for msg in stan_output:
                if msg.topic == callbacks_writer_pb2.WriterMessage.Topic.Value("SAMPLE"):
                    # Ignore sample message which is mixed together with proper draws.
                    if msg.feature and msg.feature[0].name == "":
                        continue

                    draw_row = []  # a "row" of values from a single draw from Stan C++

                    # for the first draw: collect sample and sampler parameter names.
                    if not hasattr(self, "_draws"):
                        feature_names = tuple(fea.name for fea in msg.feature)
                        self.sample_and_sampler_param_names = tuple(
                            name for name in feature_names if name.endswith("__")
                        )
                        num_rows = len(self.sample_and_sampler_param_names) + num_flat_params
                        # column-major order ("F") aligns with how the draws are stored (in cols).
                        self._draws = np.empty((num_rows, num_samples_saved, num_chains), order="F")
                        # rudimentary check of parameter order (sample & sampler params must be first)