Add docs for new gitlab-exporter server_name setting
What does this MR do?
This adds documentation for a new gitlab-exporter setting introduced in omnibus-gitlab!4896 (merged)
Does this MR meet the acceptance criteria?
Conformity
- [-] Changelog entry
-
Documentation (if required) -
Code review guidelines - [-] Merge request performance guidelines
-
Style guides - [-] Database guides
- [-] Separation of EE specific content
Related to #293687 (closed)