Add usage to gen ai OpenTelemetryChatClient

[nissbran]

Currently the OpenTelemetryChatClient support most of the attributes from the semantic conventions for gen ai 1.34.

There is a however a diff with the usage part. The gen_ai.usage.input_tokens and gen_ai.usage.output_tokens attributes are missing.

But we have gen_ai.response.input_tokens and gen_ai.response.output_tokens instead. Is this intentional or is it possible to add the usage parameters to the trace attributes?

[stephentoub]

Thanks for reporting. I'll double check, but this looks like a bug.