Add Observability group with default labels

What does this MR do and why?

This adds the Observability group to the group-definition.yml file and ensure default labels will be set for the gitlab-org/opstrace group.

Expected impact & dry-runs

• Team triage report: https://gitlab.com/gitlab-org/quality/triage-ops/-/jobs/2499160837
• Merge requests needing attention report (no MRs): https://gitlab.com/gitlab-org/quality/triage-ops/-/jobs/2499408801

See https://gitlab.com/gitlab-org/quality/triage-ops/-/tree/master/doc/scheduled#testing-with-a-dry-run on how to perform dry-runs.

Action items

• (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