feat: Include usage stats when streaming with OpenAI and ChatOpenAI

[davidmigloz]

final promptTemplate = ChatPromptTemplate.fromTemplates(const [
  (ChatMessageType.system, 'You are a helpful assistant that replies only with numbers in order without any spaces or commas'),
  (ChatMessageType.human, 'List the numbers from 1 to {max_num}'),
]);
final chat = ChatOpenAI(apiKey: openaiApiKey);

final chain = promptTemplate.pipe(chat);

ChatResult? result;
final stream = chain.stream({'max_num': '9'});
await for (final ChatResult res in stream) {
  result = result?.concat(res) ?? res;
}
expect(result!.output.content, contains('123456789'));
expect(result.usage.promptTokens, greaterThan(0));
expect(result.usage.responseTokens, greaterThan(0));
expect(result.usage.totalTokens, greaterThan(0));