Correct token calculation for streaming in case of using functions

[vladisavvv]

Currently we don't calculate tokens for functions and tools

[adubovik]

1. Consider enabling stream_options.include_usage=True
2. Keep in mind that to this day we don't how to count tokens for tools/functions presicely

[adubovik]

Just in case: there is an unofficial function tokenizer, which assumes that OpenAI converts functions to TypeScript definitions: https://github.com/rubenselander/openai-function-tokens

[adubovik]

#170 estimates token count of function by tokenizing tools config serialized to a string. It's obviously not an exact match to the OpenAI tokenization, and it may overestimate as well as underestimate the token count, but it's still better than returning 0 completion tokens on a function call.

Use with caution. The recommended way is still using stream_options.include_usage=True that provides the precise token count coming from Azure OpenAI.