Support for outputting token usage

[vazirim]

Is your feature request related to a problem? Please describe.
It would be useful to know the token usage of a program.

Describe the solution you'd like
Use the functionality in LiteLLM to surface this in PDL: https://docs.litellm.ai/docs/completion/token_usage

Describe alternatives you've considered
N/A

Additional context

This issue was proposed by @eloycoto.

[vazirim]

This information could also appear in Jaeger and in the new UI.
@esnible @starpit

[vazirim]

Double check with @esnible that we are getting this info already with open-telemetry.