Add Prometheus service discovery endpoint

[strahe]

This PR adds a Prometheus HTTP Service Discovery endpoint, which allows Prometheus to dynamically discover and scrape Curio nodes in the network.

Implements a new HTTP handler at /debug/service-discovery that returns a JSON list of targets in Prometheus SD format

Usage

Add the following job to your Prometheus configuration:

- job_name: 'curio'
    metrics_path: 'debug/metrics'
    http_sd_configs:
      - url: 'http://your-curio-node:port/debug/service-discovery'

This feature has been tested in my development environment and is working properly.