The previous commit's comments explained `_HOSTNAME` being `nixos` with
"no container sets `networking.hostName`", read off a `git grep` of this
tree. That grep answers a question about our source; the default is
nixpkgs': `nixos/modules/virtualisation/nixos-containers.nix` sets
`networking.hostName = mkDefault name` for every `containers.<name>`
guest, which is how every swarm service container here is declared.
So the stated cause is wrong, and why the default does not reach these
guests is still open. What is measured is narrower: an agent container
reports `nixos`, and `_MACHINE_ID` is per-machine by construction. The
comments now claim only that, and say the rest is unresolved.
The code is unchanged — `_MACHINE_ID` in `_stream_fields` is correct
regardless of which explanation turns out to be true.
Refs #4304
57aedc82ae