Tailscale client metrics available for Prometheus scraping

Tailscale clients expose metrics endpoint that can be scraped by Prometheus to monitor network performance, including advertised/approved routes, throughput for direct traffic and DERP relay traffic, and bandwidth usage

Enable metrics collection by running 'tailscale metrics print' to view available metrics. Configure Prometheus to scrape the Tailscale metrics endpoint. Monitor key metrics like advertised routes (tailscaled_advertised_routes), approved routes (tailscaled_approved_routes), and traffic throughput for both direct connections and DERP relay traffic