Release-trigger bot does not trigger releases properly

[losalex]

As you can see from googleapis/nodejs-logging#1281, the release was failing to trigger and only manual release re-triggering helps. Seems like a bug - please take a look

[bcoe]

Another example where I had to kick off the job manually:

googleapis/nodejs-run#2

[chingor13]

From looking at the logs, I don't see any failures. I do see the release.created events and it doesn't say it even attempted a release (there's no output of other failure cases either).

Adding some logging to output the number of found, pending release PRs. Also, adding a cron to periodically scan for the missing pull requests as a fallback. We will be able to dig in further later.

[chingor13]

I think this is a race condition between release-please creating the release (which triggers the release-trigger bot) and it labeling the PR with autorelease: tagged. This bot looks for merged PRs with the autorelease: tagged label.

#3774 should add additional logging - a release.published event log expects to see a log message that it found a release to trigger.