Support logprobsResult and avgLogprobs Response from Google Generative AI

[elikor-figma]

Feature Description

The Google Generative AI inference API allows for a user to set the responseLogprobs field to true in their request to gemini-1.5-flash, along with the optional logprobs field to specify the number of candidate logprobs in each step. When responseLogprobs is set to true, the response then includes logprobsCandidates in the response (see in the response body schema here). It additionally appears like the avgLogprobs field is returned regardless of responseLogprobs being sent, and would additionally be useful to include. I would like these fields to be exposed in some way (perhaps through provider metadata) in the request and response of the AI SDK functions using the Google Generative AI provider.

Use Cases

Logprobs can be useful for filtering and selecting different token responses, providing a means of assigning confidence to different LLM outputs.

Additional context

No response