Ensure we add a pipeline tier label when the MR opens

Context

I noticed while opening an MR that the pipeline tier label was only added when I edited the MR description, which I found too late (and was surprised by).

What does this MR do?

Add a pipeline tier label when the MR opens.

Will it also be reflected in the pipeline name?

I don't think so. I expect the set of labels passed to the first pipeline to be the ones present when the MR was initially created (related to gitlab-org/gitlab!154372 (merged)).

Nevertheless, I think it's conceptually nicer to have this label being applied when the MR opens.

Expected impact & dry-runs

Not a big impact is expected.

Action items

• If adding environment variables for reactive processors, update config/triage-web.yaml and .gitlab/ci/triage-web.yml
• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review