Implemented User Principal Tag in API metrics with readiness checks for the renovate-bot/apache-_-polaris repository (August 2025). This feature adds user context to API metrics while introducing production readiness warnings when enabling the tag in conjunction with the realm ID tag. Also updated metrics configuration and expression resolution to fully support the new tag, tied to commit 3f1dfb332b84e0eeeab5411b333883790273f504 ("Add user principal tag in metrics (#2445)"). No major bugs fixed this month; the work focused on delivering a safe, observable metric tagging enhancement. Business value includes improved observability, security posture, and informed rollout decisions. Technologies demonstrated: metrics instrumentation, configuration management, expression resolution, and production readiness checks.