outputs.prometheus_client: vsock listener silently ignores a CID in the listen address

[skartikey]

Relevant telegraf.conf

[[outputs.prometheus_client]]
  ## Run on a guest whose context ID is 3, but name the host CID
  listen = "vsock://2:9273"

Logs from Telegraf

2026-09-22T09:00:00Z I! [outputs.prometheus_client] Listening on http://vm(3):9273/metrics

The log line shows the bound context ID, but the address is accepted and no warning says the configured CID was dropped.

System info

Telegraf 1.40.1, Linux with vsock

Docker

No response

Steps to reproduce

1. Run Telegraf in a VM whose context ID is not the one in the address, e.g. a guest with CID 3.
2. Configure listen = "vsock://2:9273", so the CID names the host rather than the guest.
3. Start Telegraf and check what the listener bound, e.g. ss -l --vsock.

Expected behavior

Either the listener binds the CID given in the address, or Telegraf rejects the address at startup because that CID is not ours. The plugin knows both before any scrape arrives.

Actual behavior

listenVsock in plugins/outputs/prometheus_client/prometheus_client.go splits the address with net.SplitHostPort and discards the host half, then calls vsock.Listen(port, nil), which binds the context ID of the machine Telegraf runs on. The listener in the example comes up on CID 3 while the config says 2, and a scraper addressing CID 2 never reaches it.

Additional info

The documented form is listen = "vsock://:9273" with no CID, and for that form binding the local context ID is correct, so this only bites the undocumented spelling that carries one. Honoring it with vsock.ListenContextID when the host part is non-empty would match inputs.socket_listener (#19751) and outputs.socket_writer, which passes the CID to vsock.Dial. A CID that is not ours would then fail at startup with EADDRNOTAVAIL rather than binding something else, which is the behavior change to call out in the CHANGELOG.

Same class as #19751, but a separate code path and a separate decision, since here the empty CID is the documented default.