Update OpenAI::Client cost tracking to include feature category

What does this MR do and why?

This MR addresses additional feedback raise in !118925 (comment 1372532439) by adding feature_category information. It also updates item value which previously included model information, but it's now removed, in order to reduce metric cardinality and only have 2 values: prompt & completion.

Mentions https://gitlab.com/gitlab-org/gitlab/-/issues/407334

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