Log how many tokens used in Langchain

[rodrigorodriguez]

import { ChatOpenAI } from 'langchain/chat_models/openai';

let totalCompletionTokens = 0;
let totalPromptTokens = 0;
let totalExecutionTokens = 0;

const llm = new ChatOpenAI({
callbacks: [
{
handleLLMEnd: (output, runId, parentRunId?, tags?) => {
const { completionTokens, promptTokens, totalTokens } = output.llmOutput?.tokenUsage;
totalCompletionTokens += completionTokens ?? 0;
totalPromptTokens += promptTokens ?? 0;
totalExecutionTokens += totalTokens ?? 0;
},
},
],
modelName: 'gpt-3.5-turbo-0613',
});