Port PR from PBS-Go: Metrics: Record incoming request sizes

[cross-repo-issue-creation]

A PR was merged over on PBS-Go

• Metrics: Record incoming request sizes prebid-server#4495
• timestamp: 2025-09-04T21:01:22Z

[Net-burst]

PBS-Java already has those metrics. The request size metric is not as detailed though. Although if the community would prefer the more detailed request size metric, we would gladly accept the contribution. Will close this for now.