pass model and cost so that langfuse can show cost

[obadakhalili]

Summary

Dify's integration with Langfuse doesn't show generation cost.

Closes #10684 (and probably more similar issues).

Details

The code that transforms Message into dict to attach as message_data in trace_info object doesn't include total_price and model_id, which are necessary for Langfuse to calculate the cost. The same is true about the code responsible for instantiating a Message object from a dict.

Some things to be aware of:

• I can pass the model ID alone, and Languse will use this with the rest of the available information to infer the cost, but I thought that it would be better to pass the cost if it is available. Should this be reconsidered?
• usage_details is deprecated

Screenshots

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After:

Checklist

Important

Please review the checklist below before submitting your pull request.

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods