Prompt and completion tokens (including corresponding prometheus metrics) not tracking when streaming chat client's response

The below returns the prompt and completion tokens

ChatClient.CallResponseSpec responseSpec = _chatClient.prompt("Tell me a dad joke about dogs")
            .call();
        ChatResponse chatResponse = responseSpec.chatResponse();
        ChatResponseMetadata chatResponseMetadata = chatResponse.getMetadata();

        int promptTokens = chatResponseMetadata.getUsage().getPromptTokens();
        int completionTokens = chatResponseMetadata.getUsage().getCompletionTokens();
        int totalTokens = chatResponseMetadata.getUsage().getTotalTokens();

        _LOGGER.debug("Prompt tokens: {}", promptTokens);
        _LOGGER.debug("Completion tokens: {}", completionTokens);
        _LOGGER.debug("Total tokens: {}", totalTokens);

But streaming the chat client's response doesn't

ChatClient.StreamResponseSpec streamResponseSpec = _chatClient.prompt("Tell me a dad joke about dogs")
            .stream();

        Flux<ChatResponse> chatResponseFlux = streamResponseSpec.chatResponse();
        chatResponseFlux.map((r) -> {
            ChatResponseMetadata chatResponseMetadata = r.getMetadata();
            int promptTokens = chatResponseMetadata.getUsage().getPromptTokens();
            int completionTokens = chatResponseMetadata.getUsage().getCompletionTokens();
            int totalTokens = chatResponseMetadata.getUsage().getTotalTokens();

            _LOGGER.debug("Prompt tokens: {}", promptTokens);
            _LOGGER.debug("Completion tokens: {}", completionTokens);
            _LOGGER.debug("Total tokens: {}", totalTokens);
        });

Start with the ChatClient.CallResponseSpec and StreamResponseSpec entry points shown in the issue, comparing how chatResponse() exposes metadata for non-streaming and streaming calls. Trace the streaming response handling and corresponding Prometheus metrics; done means prompt, completion, and total token usage are available and tracked during streaming, with regression coverage for that path.