Use new prometheus metrics URL to avoid an extra redirection

What does this MR do?

Update new prometheus metrics URL to avoid an extra redirection like the previous fix at gitlab-foss!23790 (merged).

• Old URL: https://docs.gitlab.com/ee/user/project/integrations/prometheus_library/metrics.html
• New URL: https://docs.gitlab.com/ee/user/project/integrations/prometheus_library/index.html

Replaces !8836 (closed)

Does this MR meet the acceptance criteria?

Conformity

• [n/a] Changelog entry
• Documentation created/updated or follow-up review issue created

Closes #31953 (closed)