Extract Alertmanager service from monitoring service
## Overview The scope of this issue is yet to be defined, but it's created to track the work from https://gitlab.com/groups/gitlab-com/gl-infra/-/epics/630 about extracting Alertmanager as it's own service from the Monitoring service. We'll start with production-engineering~13296694 first in https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/14643
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