Motivation for this change

Alertmanager fails to start when the memberlist library cannot find any private IP addresses on the system. This is a safeguard to avoid exposing cluster ports over the internet unintentionally. The workaround is to pass the --cluster.advertise-address argument to Alertmanager.

See also:
prometheus/alertmanager#1437
prometheus/alertmanager#1434

FWIW, I've deployed this to a machine managed by NixOps and it seems to work (with clusterAdvertiseAddress = "${config.deployment.targetHost}:9094";)

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Fits CONTRIBUTING.md.