[metrics] add deployment_rollbacks_started_total metric
What does this MR do?
In this MR we are adding a new metric deployment_rollbacks_started_total to keep track of started rollbacks.
Rollback tracking will be implemented in chatops.
Related to gitlab-com/gl-infra/delivery#1952 (closed)
Edited by Alessio Caiazza (OOO until 2024-08-26)