feat(ci): add missing feat label checker

[vishwarajanand]

Addresses #6397

[bshaffer]

Do we have any PRs where we've tested this?

This can be done by opening a new PR from a branch forked from this one, labeled feat instead of chore but with a new class or method added. This check should fail.

[vishwarajanand]

Do we have any PRs where we've tested this?

Actually, no. Because we need an owlbot PR to test this, so I could not raise one.

[bshaffer]

we need an owlbot PR to test this

You can just comment out this line in the test PR and then you can test it manually.

[vishwarajanand]

Hi @bshaffer, thanks for the tip.

I performed the tests and wrote the results here: #6481 (comment)

Following up on the tests, I made following changes to the PR:

1. Added types because its supposed to re-run & pass (before reviewers can merge affected PRs) when PR title is changed to feat
2. Ran the BC compat checker from github.event.pull_request.head.sha to steps.latest-release.outputs.release, because it more accurately represents the latest changes included in the PR
3. startsWith will change to !startsWith
4. Removed continue-on-error

One thing to note is that, when a new function is added the failure message says function is removed instead of added:
Error: Method Google\Cloud\Firestore\FirestoreClient#myCustomFunction() was removed

[bshaffer]

when a new function is added the failure message says function is removed instead of added

Yes, I think we may want to do some bash foo here to catch the exception message and throw a different one. And we can get rid of the PR annotations (by removing --format=github-actions from the command) because we don't need them for this - we just want a pass/fail

[vishwarajanand]

@bshaffer I added just a single print statement and pass/fail for that, PTAL
A failure looks like this in PR#6481