feat: add weekly metrics reporting workflow

[nearlyforget]

Description

This PR introduces the metrics.yml workflow to automate the collection and reporting of weekly repository activity. It leverages a reusable workflow from the .github
repository to track metrics for Issues, Pull Requests, and Discussions.

Key Features:

• Automated Reporting: Scheduled to run every Monday at 02:00 UTC.
• Manual Trigger: Supports workflow_dispatch for on-demand report generation.
• Issue Creation: Automatically generates a markdown-formatted issue in this repository summarizing response times, close times, and activity volume.

Category (Required)

• Core Protocol
• Governance/Contributing
• Capability
• Documentation
• Infrastructure: CI/CD, Linters, or build scripts.
• Maintenance
• SDK
• Samples / Conformance

Related Issues

Checklist

• I have followed the [Contributing Guide (https://github.com/agentic-commerce/ucp/blob/main/CONTRIBUTING.md).
• I have updated the documentation (if applicable).
• My changes pass all local linting and formatting checks.
• (For Core/Capability) I have included/updated the relevant JSON schemas.
• I have regenerated Python Pydantic models by running generate_models.sh under python_sdk.