Add alerts routing for observability team to observability's Slack channel

Pawel Rozlachrequested to merge
prozlach/add_observability_alerting_p1 into master

Add alerting configuration for observability team as part of gitlab-org/opstrace/opstrace#1924 (closed)

Dead Mans Snitch will be added later once https://gitlab.com/gitlab-com/gl-infra/reliability/-/issues/16879 is addressed. Pagerduty will be added later once https://gitlab.com/gitlab-com/team-member-epics/access-requests/-/issues/19140 is addressed.

Merge request reports