Add ErrorRate SLI to Global Search searches

What does this MR do and why?

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

How to set up and validate locally

1. Search anything via global search /search endpoint or search api
2. Check prometheus, it should now have gitlab_sli_global_search_total and gitlab_sli_global_search_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 #342069 (closed)