Track token usage for AI Providers

What does this MR do and why?

Adds tracking to AI features with an approximated measurement of our token usage for Anthropic and Vertex. This enables us to group token usage per feature or per user.

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/415944

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.