Prompt Events Redundantly Count Tokens

[collindutter]

We calculate token_count as part of StartPromptEvent and FinishPromptEvent Events here.

This can be improved in the following ways:

• For StartPromptEvent, we should calculate this value once in the BasePromptDriver and then pass the value to subclasses.
• For FinishPromptEvent, many of the LLMs will return the token count in the response. We should not calculate it ourselves unless the value is missing.