Port PR from PBS-Go: Metrics: Record GVL fetch count

[cross-repo-issue-creation]

A PR was merged over on PBS-Go

• Metrics: Record GVL fetch count prebid-server#4500
• timestamp: 2025-09-04T14:55:35Z

[AntoxaAntoxic]

PBS Java already has metrics like privacy.tcf.{v1,v2}.vendorlist.{ok,err} which record the count of vendor list updates.