I stumbled upon an issue with the Alertmanager that required
an additional comand line option. See https://groups.google.com/forum/#!msg/prometheus-users/-5wd-P13xCI/lGLBHHgnBgAJ

(cherry picked from commit 69e4e49)

Without this, Nixos machines on 18.09 that have no private IP addresses cannot start Alertmanager.
#46068
#45302

Motivation for this change

I am trying to start Alertmanager on my machine, which does not have a private IP address (only a public IP address, and localhost). As described in #46068 , to do this requires setting --cluster.advertise-address, otherwise Prometheus fails to start.

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)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.