LoggingBunyan: should write diagnostic entry failed

[flaky-bot]

Note: #727 was also for this test, but it was closed more than 10 days ago. So, I didn't mark it flaky.

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commit: d9b6b2f
buildURL: Build Status, Sponge
status: failed

[flaky-bot]

Looks like this issue is flaky. 😟

I'm going to leave this open and stop commenting.

A human should fix and close this.

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When run at the same commit (d9b6b2f), this test passed in one build (Build Status, Sponge) and failed in another build (Build Status, Sponge).

[cindy-peng]

Looks like flaky tests: https://fusion2.corp.google.com/invocations?q=target:cloud-devrel%2Fclient-libraries%2Fnodejs%2Fnightly%2Fgoogleapis%2Fnodejs-logging-bunyan%2Fnode14%2Fsystem-test
Tests have been passing for last week.