plugin/label: Improve error when adding/removing a restricted label

[stephenfin]

Currently, if you attempt to add a restricted label and allowed_teams is unset/empty, you get the following unhelpful message.

@username: Can not set label cherry-pick-approved: Must be member in one of these teams: []

Improve this so that we no longer suggest being a member of no team and instead suggest reaching out to one of the users in allowed_users instead:

@username: The label(s) `restricted-label` cannot be applied or removed, because you are not in one of the allowed teams and are not an allowed user. Consider assigning one of the following members: Mallory

We also improve the common path where allowed_teams is populated, aligning the error message with those of other errors paths:

@username: The label(s) `restricted-label` cannot be applied or removed, because you are not in one of the allowed teams and are not an allowed user. Must be a member of one of these teams: privileged-group

Note

I suspect dumping a list of users could get "chatty", so I've intentionally avoided formatting these as links (i.e. we emit user, not @user) and only print up to 3 users. In most cases, this code should not be triggered since projects will usually make use of allowed_teams.

Note

I'm still not entirely happy with this either. For example, see here. That error messages suggests only a member of openshift-staff-engineers can set the label, when in reality there are many users that can set it. Perhaps we should reword this as either-or ("you must be a member of these groups: xyz. Alternatively, consider assigning one of these members: abc"). Open to idea here.

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[stephenfin]

/test pull-prow-integration

That...doesn't look related...

[k8s-ci-robot]

@stephenfin: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-prow-unit-test-race-detector-nonblocking	d77cd16	link	false	/test pull-prow-unit-test-race-detector-nonblocking

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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[stephenfin]

/retest-required

pull-prow-unit-test-race-detector-nonblocking is failing on pkg/interrupts/interrupts_test.go:183, which I'm not touching here.

pull-prow-integration failed on the same test, but again this doesn't look related

[smg247]

I see no reason why the message can't display the allowed teams and (a subset of) the allowed users as well. You can format a GH message so that it hides content by default to be expanded, maybe if the list is longer than 5 or so you can put an ellipsis, and hide the rest behind it. I am not sure that I would bother with returning random users. In situations where there are 20+ users, maybe just return the first 20... WDYT?