Support BYO Elastic Stack instead of GMAv1 Elastic Stack

Similar to #292460 (closed) but for Elastic Stack

Proposal

Just provide a simple, checkbox-like experience. The application is assumed to be installed with Helm, in a fixed namespace (gitlab-managed-apps), and with a specific name.

If the user enables the integration, we mark it as Externally installed in the appropriate model. Everything else that depends on that model should continue to work as normal.

Project cluster "cluster name"

Details | Health | Integrations | Advanced Settings

[x] Enable ElasticStack integration.

Enables ElasticStack integration, via Kubernetes API. ElasticStack must be installed in the Kubernetes cluster, a Service resource called elastic-stack-elasticsearch-master must exist in the gitlab-managed-apps namespace so that GitLab may use that Service to make Elasticsearch queries. Learn more

Release notes

By integrating your cluster services with GitLab you can benefit from various GitLab features, like Environment boards, Prometheus metrics, and application logs. Previously, these features required you to use GitLab Managed Apps which did not suit the workflow and requirements of many of our users.

With this release, you can integrate ElastiSearch through GitLab services and keep their maintenance on your end, following your own company processes and policies. We provide extensive documentation and a recommended workflow on how to install these applications if you are just getting started. You can still hold the deep metrics integrations available in GitLab as you had with GitLab Managed Elastic Stack.