pull-npd-e2e-test failing ssh handshake

https://testgrid.k8s.io/presubmits-node-problem-detector#pull-npd-e2e-test starts to fail recently.
```
[1] NPD should export Prometheus metrics. When OOM kills and docker hung happen
[1] NPD should update problem_counter and problem_gauge
[1] /home/prow/go/src/k8s.io/node-problem-detector/test/e2e/metriconly/metrics_test.go:158
[2] error dialing prow@35.184.209.153:22: 'ssh: handshake failed: read tcp 10.32.2.7:54804->35.184.209.153:22: read: connection reset by peer', retrying
[2] error dialing prow@35.184.209.153:22: 'ssh: handshake failed: read tcp 10.32.2.7:52980->35.184.209.153:22: read: connection reset by peer', retrying
[2] error dialing prow@35.184.209.153:22: 'ssh: handshake failed: read tcp 10.32.2.7:53002->35.184.209.153:22: read: connection reset by peer', retrying
[2] error dialing prow@35.184.209.153:22: 'ssh: handshake failed: read tcp 10.32.2.7:44696->35.184.209.153:22: read: connection reset by peer', retrying
[2] Error storing debugging data to test artifacts: [Error running command: {prow 35.184.209.153 curl http://localhost:20257/metrics 0 error getting SSH client to prow@35.184.209.153:22: 'ssh: handshake failed: read tcp 10.32.2.7:52990->35.184.209.153:22: read: connection reset by peer'}
[2] Error running command: {prow 35.184.209.153 sudo journalctl -u node-problem-detector.service 0 error getting SSH client to prow@35.184.209.153:22: 'ssh: handshake failed: read tcp 10.32.2.7:44688->35.184.209.153:22: read: connection reset by peer'}
[2] Error running command: {prow 35.184.209.153 sudo journalctl -k 0 error getting SSH client to prow@35.184.209.153:22: 'ssh: handshake failed: read tcp 10.32.2.7:44708->35.184.209.153:22: read: connection reset by peer'}
[2] ]
```

This is affecting several different PRs: https://github.com/kubernetes/node-problem-detector/pull/955, https://github.com/kubernetes/node-problem-detector/pull/961, https://github.com/kubernetes/node-problem-detector/pull/969.

Start with test/e2e/metriconly/metrics_test.go at line 158 and reproduce the pull-npd-e2e-test failure from the linked TestGrid job. Inspect the SSH access used for the prow host and the commands that collect metrics and journal logs. Done means the e2e test completes without the SSH handshake failure and debugging data can be stored.

索引モデルが issue の本文から書いたものです。