Expose global_search_apdex as a prometheus metric

What does this MR do and why?

This MR creates custom SLIs for Global Search apdex for Web and API.

• Implement different duration successes for different searches - we need the new service levels
• Update docs page: https://docs.gitlab.com/ee/development/application_slis/#existing-slis

How to set up and validate locally

1. Search anything via global search /search endpoint
2. Check prometheus, it should now have gitlab_sli_global_search_web_apdex_total and gitlab_sli_global_search_web_apdex_success_total

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.

Related to #342068 (closed)