Add basic SLI for AI chat

What does this MR do and why?

Adds a custom SLI for measuring success of AI chat responses.

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/419629

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

• enable duo chat - https://docs.gitlab.com/ee/development/ai_features.html#set-up-gitlab-duo-chat
• execute a chat question synchronously (e.g. from rails console)
• observe in metrics (http://:3000/-/metrics) that the relevant counter was incremented

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.