[add] Prometheus exporter

[shinya-dono]

Why

since backhaul does not support multi-server it's usually required to have multiple backhaul instances, and overtime, managing and monitoring all those instances gets harder and harder.

the current monitoring solution is not enough for multiple servers and is incompatible with most modern monitoring tools. (as well as eventual timeouts due to how the network speed is calculated)

this PR adds /metric endpoint for prometheus scraping as well as a basic abstraction for future monitoring modules.
together with node exporter it would be real easy to have all backhaul nodes in one dashboard at one glance.

Description

• Added support for Prometheus exporter to enable monitoring capabilities.
• Isolated metrics states into module global state for better modularity and maintainability.
• Provided an example dashboard and Prometheus/Grafana setup for demonstration.
• Updated README.md to include usage examples.

Breaking changes

• none, this PR introduces no breaking change

Note

the legacy monitoring system is kept as default, without specifying metrics parameter in config or an empty array, it will fall back to the legacy metrics system.