Add usage tracker to GPT by price

[leordev]

Three caveats here:

• The pricing per token usage is hardcoded from what I got in the OpenAI website today
• The calculation of the stream chunks is an approximate calculation using tiktoken (official tokenizer by OpenAI)
  • This is a super requested feature as you can see here: https://community.openai.com/t/openai-api-get-usage-tokens-in-response-when-set-stream-true/141866/1
• We check for the cost limit before the prompts execution, so if its a huge request prompt it might pass the limit by a few cents.

[michaelneale]

no permissions to push - but otherwise looks ok. do you want to keep this @leordev or wait for the new langchain flavor?

[leordev]

The no permissions to push is because it's a forked pr, then github does not expose the GH token secret. you can go ahead and merge. So we can get rid of your OpenAI Key while we dont merge the new langchain stuff.