Grafana metrics dashboard generator for all Lighthouse metrics

[jimmygchen]

Issue Addressed

• Maintaining the separate [lighthouse-metrics](https://github.com/sigp/lighthouse-metrics) repository has become a pain
• We now have 437+ metrics across the codebase and dashboard creation is currently ad-hoc, and we don't have them version controlled.

This PR adds a ``metrics_dashboard_generator` binary that

• Automatically scans the Lighthouse codebase for all Prometheus metrics
• Parses metric definitions from metrics.rs files across all crates
• Generates a comprehensive Grafana dashboard JSON with panels organized by crate

I've labelled this as do-not-merge because I'm not sure if we want to officially maintain this, but it may be a useful tool to experiment with.

Potential improvements:

• Infer the correct unit from the metric name

Usage

cargo run --bin metrics_dashboard_generator

This eliminates the need to manually maintain dashboard configurations and ensures all metrics are discoverable and visualized automatically as the codebase evolves.

Output from unstable

[jimmygchen]

Closing this PR as this is not something we will maintain.