feat: support multiple metrics addresses via repeated flag

[IngmarStein]

Summary

This PR updates the --metrics flag to accept multiple values, allowing the metrics server to bind to specific addresses (e.g., localhost IPv4 and IPv6) without exposing the endpoint to all interfaces via [::].

Changes

• CLI: Changed metrics flag to StringSliceFlag. Users can now pass --metrics multiple times.
• Metrics Package:
  • CreateMetricsListener now accepts a slice of addresses and returns a slice of listeners.
  • ServeMetrics now accepts a slice of listeners and serves them concurrently using a shared http.Server.
• Tunnel Command:
  • StartServer updated to initialize and close multiple listeners.
  • readyCommand updated to check the first configured metrics address.
• Tests: Added test cases for multiple listener creation.

How to test

Run the tunnel with multiple metrics flags:

cloudflared tunnel run --metrics 127.0.0.1:2020 --metrics [::1]:2020 ...

Verify that the metrics endpoint is accessible on both addresses but not on external interfaces.

[jcsf]

Hi,
What is the problem you are trying to resolve here? Why is more than 1 metrics address needed? Can you give us an example or a more detailed explanation?

[IngmarStein]

@jcsf if I use "--metrics 127.0.0.1:2020", cloudflared only listens on IPv4 (at least on macOS 26) and with "--metrics [::1]:2020", it listens on IPv6 only. I'd like to make the metrics endpoint available on both stacks, but without opening it up beyond localhost.