[Analytics] Feature request: ability to filter out PRs with specific label(s)

[zcorpan]

For https://github.com/web-platform-tests/wpt there are a high number of PRs that are exported from downstream projects (like Chromium, Gecko, Servo), that are automatically approved by a bot.

It would be nice to be able to exclude those PRs from the analytics. Filtering based on excluding labels would work for our case.