Port PR from PBC-Java: Emit unauthorized writes metric

[cross-repo-issue-creation]

A PR was merged over on PBC-Java

• Emit unauthorized writes metric prebid-cache-java#154
• timestamp: 2025-02-19T23:54:28Z

[bretg]

A metric was added in PBC-Java called pbc.write.trustedRequest, which logs when a write comes in with an API key. This allows host companies that are utilizing api.api-key configuration to get a sense for the percentage of writes that are coming in from Prebid Server vs external sources like Prebid.js.

This can be useful in determining whether external writes can be turned off safely.