o11y: change the metric name to llm_model_cost_total and add a currency label

[tao12345666333]

Your suggestion is good.
Maybe I can change the metric name to llm_model_cost_total and add a currency label. e.g. llm_model_cost_total{currency="USD"}

But a trade-off needs to be made here.

Pros:

• Easy to extend, allowing us to add CNY, EUR, etc, in the future as needed
• Clearer. A metric name for "cost," with currency as the dimension

Cons:

• Aggregations must be done per-currency (sum by (currency, model) …). It’s easy to accidentally sum different currencies if you forget to filter.

WDYT? @Xunzhuo @rootfs

Originally posted by @tao12345666333 in #66 (comment)

[tao12345666333]

I will fix this soon. Please assign it to me @rootfs