Client-side prompt token count is inaccurate

[sjmonson]

The client-side tokenization in guidellm fails to account for the extra tokens added in the server's chat prompt template. There are two possible workarounds:

1. Enable usage metrics in each request and let the server tell us how many prompt tokens there are.
2. Use the /completions endpoint rather than /chat/completions as the chat template is not applied on the /completions endpoint.

[markurtz]

#91 contains fixes towards both. There will be a bit of follow up work past that to enable the user further configuration on calculations and selecting which end point to request to