adding get_num_tokens for langchain's anthropic client

[Ilaiwi]

Checked

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Feature request

The langchain anthropic client doesn't have get_num_tokens implemented. Recently Anthropic added this API to expose a way to count tokens.

Motivation

I have a use-case where I want to know what are the number of tokens in my messages array and depending on it, I decide to use different text to pass to the llm if the token count exceeded X tokens

Proposal (If applicable)

This should be stright forward, the only concern I have that get_num_tokens is not a Continuous function. The api should be fast but it would be cool if it's not blocking my asyncio loop.

[Ilaiwi]

actually get_num_tokens_from_messages already has this method. My bad.