[RFC] Allow intention management with labels

[webknjaz]

Idea

Project maintainers should be able to set labels instructing the bot to omit or enforce requiring change fragments.

Implementation

If bot:chronographer:skip label is set, the bot should report a neutral status.
Otherwise, the bot should set bot:chronographer:required label if the fragment is missing and bot:chronographer:provided if it's there.
The bot should allow maintainers to set bot:chronographer:more if the fragment is present but needs to be updated/extended or maybe if the PR needs several fragments. The bot should report "action_required". Once the requirement is fulfilled, the bot must remove this label.

Ref: tox-dev/tox#1364 (comment)

[webknjaz]

@asottile @gaborbernat @asvetlov does above look reasonable?

[gaborbernat]

Sounds good to me 👍

[asvetlov]

1. bot:chronographer:required is 25-chars line. Does it fit into GitHub UI well?
2. I don't understand the purpose of bot:chronographer:more. If the CHANGELOG record is incorrect -- reviewer points on it in review message, that's it. How does bot determine that the label should be removed?
3. What is the purpose of adding bot:chronographer:provided label? To use it in PR filers?

[webknjaz]

Does it fit into GitHub UI well?

LGTM:

How does bot determine that the label should be removed?

I think on the change to that file or adding a new one.

To use it in PR filers?

Yep, more or less.