feat(observability): Add container memory utilization

[nfebe]

Add a container memory utilization metric so percentage alert thresholds use each container's effective memory limit.

[sourceant]

Code Review Summary

This PR introduces a new container.memory.utilization metric (calculated as usage/limit * 100). This allows users to set percentage-based alerts that are relative to each container's specific memory constraints. The implementation is consistent across the entire observability stack, including storage, alerting, OTLP, and Prometheus exporters.

🚀 Key Improvements

• Introduced MetricMemoryUtilization calculation in the storage layer with safety checks for zero limits.
• Unified formatting for percentage-based metrics in alert messages.
• Full integration with OTLP and Prometheus exporters including proper units (%) and help documentation.
• Comprehensive test coverage across API, storage, and alert formatting layers.

[sourceant]

Review complete. No specific code suggestions were generated. See the overview comment for a summary.