[ECR] [request]: Add CloudWatch metrics for number of tags per repository per region

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**Tell us about your request**
Request to add CloudWatch metrics that show the number of tags for a given ECR repository in a specific region.

**Which service(s) is this request for?**
Amazon Elastic Container Registry (ECR)

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
Currently, there's no straightforward way to monitor the growth of image tags within ECR repositories. This metric is crucial for:
- Monitoring repository growth and managing costs
- Setting up automated alerts when repositories exceed certain tag thresholds
- Capacity planning and cleanup scheduling

**Additional context**
This feature would complement the existing FR #578 for repository count and image count metrics providing a complete view of ECR resource usage. The metric would be particularly valuable for organizations managing multiple repositories with automated CI/CD pipelines that generate numerous tags.

Start by reviewing the request for CloudWatch metrics on ECR repository tags and the related FR #578 for repository and image count metrics. Define the metric scope for a repository in a specific region, including how it supports growth monitoring, alerting, capacity planning, and cleanup scheduling; done means the requested feature is specified and available.

Written by the indexing model from the issue text.