Feature Request: Add metric to monitor ResourceGroup object size against etcd limits

### Checklist

- [x] I did not find a related open enhancement request.
- [x] I understand that enhancement requests filed in the GitHub repository are by default low priority.
- [x] If this request is time-sensitive, I have submitted a corresponding issue with [GCP support](https://cloud.google.com/support-hub).

### Describe the feature

Description: Currently, users struggle to know precisely how close a ResourceGroup object is to hitting the Kubernetes API server or etcd object size limits. While metrics like resource_count exist, they serve only as a rough heuristic and do not accurately reflect the actual serialized size of the object. If the limit is reached, it causes synchronization failures that are hard to predict.

Feature request: We would like Config Sync to surface a new metric that tracks the size of the serialized ResourceGroup object (or an estimate of how close it is to the limit). This would allow users to monitor their usage and proactively restructure their configurations (e.g., by splitting repositories) before hitting hard limits.

Alternatives considered: Monitoring the declared_resources or resource_count metrics. However, since individual resource sizes vary significantly, a simple count is often an insufficient proxy for the actual object size.

### Importance

_No response_