feat: run HTTP server standalone without Kubernetes

[tobiasgiese]

fixes #1332

Extract /healthz, /conditions and /debug/pprof into a new httpexporter package so those endpoints are available when --enable-k8s-exporter=false. Previously the HTTP server was started only inside the k8s exporter, making it unavailable in non-Kubernetes environments. The new httpexporter tracks conditions in-memory via ExportProblems and starts on --port regardless of whether the Kubernetes API server is reachable.

[kubernetes-prow]

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[tobiasgiese]

With a local built image I can see

❯ docker run --rm --name npd --privileged -v /dev/kmsg:/dev/kmsg:ro gcr.io/k8s-staging-npd/node-problem-detector:v1.37.0-alpha.1-31-g7bff5a4c-dirty \
  --enable-k8s-exporter=false \
  --prometheus-port=20257 \
  --config.custom-plugin-monitor=config/custom-plugin-monitor.json

❯ docker exec -ti npd bash
root@5bcbcdda7f64:/# apt update -qq; apt install -qq -y curl jq &>/dev/null
2 packages can be upgraded. Run 'apt list --upgradable' to see them.
root@5bcbcdda7f64:/# curl -s http://127.0.0.1:20256/conditions | jq
[
  {
    "type": "ReadonlyFilesystem",
    "status": "False",
    "transition": "2026-08-03T12:41:01.208945846Z",
    "reason": "FilesystemIsNotReadOnly",
    "message": "Filesystem is not read-only"
  },
  {
    "type": "KernelDeadlock",
    "status": "False",
    "transition": "2026-08-03T12:41:01.208911429Z",
    "reason": "KernelHasNoDeadlock",
    "message": "kernel has no deadlock"
  },
  {
    "type": "XfsShutdown",
    "status": "False",
    "transition": "2026-08-03T12:41:01.208911554Z",
    "reason": "XfsHasNotShutDown",
    "message": "XFS has not shutdown"
  },
  {
    "type": "CperHardwareErrorFatal",
    "status": "False",
    "transition": "2026-08-03T12:41:01.208911596Z",
    "reason": "CperHardwareHasNoFatalError",
    "message": "UEFI CPER has no fatal error"
  },
  {
    "type": "NTPProblem",
    "status": "Unknown",
    "transition": "2026-08-03T12:41:01.214200471Z",
    "reason": "NTPIsUp",
    "message": "Could not find 'systemctl' - require systemd"
  }
]