Add OCI metrics to the repo server

Add OCI metrics to the repo server: a task in LegoFlow-SWE (Harbor dataset). Currently, when Helm charts are stored in OCI registries, the repo server does not expose any metrics for OCI operations. Git operations are already monitored via the argocd git family of metrics, but there is no…

The task

Currently, when Helm charts are stored in OCI registries, the repo server does not expose any metrics for OCI operations. Git operations are already monitored via the `argocd_git_*` family of metrics, but there is no equivalent for OCI. This makes it impossible to track OCI request rates, latencies, and failures.

Part of Lego-X/LegoFlow-SWE.