Check Gemini response success before trying to parse usage metadata

[andyw8]

If the API key is missing, Gemini will respond with:

{
  "error": {
    "code": 403,
    "message": "Method doesn't allow unregistered callers (callers without established identity). Please use API Key or other form of API consumer identity to call this API.",
    "status": "PERMISSION_DENIED"
  }
}

so response["usageMetadata"]["promptTokenCount"] will raise an exception about a method being called on nil.

(there may be use cases where there is an error but usageMetadata is still available. If so, then a check for that key's presence would be better).

[andyw8]

(similar issues may be present for other providers).

[swerner]

Thanks @andyw8!

[swerner]

(similar issues may be present for other providers).

Yeah probably...I had been putting this change off as I was thinking about a better way to handle logging...potentially even creating a Provider base class...but guess I'd put it off too long...