[Feature Request] Add token metrics to tracing for Non-OpenAI models

Is your feature request related to a problem? Please describe.
I am currently running experiments (flex flows) with Phi-3 mini and want to track the number of prompt and completion tokens as part of my experiments. The tracing view that shows tokens per call and total number of tokens for OpenAI is great, and I'd love to see an easy way to integrate other models with this.

Describe the solution you'd like
A clear method or documentation on how to track token consumption for calls to non-OpenAI models like Phi-3. In this case, I think it is expected that the application code must calculate the number of tokens, but there should be a helper method for formatting that token information into the proper format and sending it to the tracing UI.

Describe alternatives you've considered
Calculating the token usage as part of the flow and adding them as fields to the flow output response. This way they will show up as columns (and I can use an aggregate method to summarize), but it also exposes that information to any consuming application when I eventually deploy.

No files or tests are named. Start by locating the existing OpenAI token-tracing integration and the tracing UI's token fields; determine how non-OpenAI callers can provide prompt and completion counts without exposing them in flow outputs. Done means a documented or supported helper path records those metrics in the tracing view.