Add OpenAI token counting

[DouweM]

Description

• Docs:
  • https://cookbook.openai.com/examples/how_to_count_tokens_with_tiktoken#6-counting-tokens-for-chat-completions-api-calls
  • https://github.com/openai/tiktoken
• count_tokens is currently supported on GoogleModel and BedrockModel
• works with UsageLimits.count_tokens_before_request

References

No response

[pydanticai-bot]

PydanticAI Github Bot Found 1 issues similar to this one:

1. "Add Anthropic token counting #3429" (90% similar)

[wirthual]

Can I help with this?

[DouweM]

@wirthual Please do! Let me know if you'd like some guidance on implementation.

[vgrbuilds]

I would love to contribute

[wirthual]

@DouweM Yes, I created a PR with a test case, would be great if you can have a look.

It somehow has a problem with casette files which I could fix locally adding --record-mode=once when invoking pytest.