Add etcd integration for Elastic Agent

[Alphayeeeet]

Describe the enhancement:
The etcd module (currently only available for standalone Metricbeat deployments) should be available for Elastic Agent as a Fleet integration.
https://www.elastic.co/guide/en/beats/metricbeat/8.11/metricbeat-module-etcd.html

Describe a specific use case for the enhancement or feature:
When monitoring a Kubernetes cluster, you can already get metrics of control plane components like kube-scheduler or controller-manager. Etcd as the central database, where all cluster objects are stored, should be also available to monitoring via elastic agent.

What is the definition of done?
Added an integration to deploy via fleet, so etcd-metrics can be collected using Elastic Agent.

This root comment is resolved. Please take a look at this one: #8885 (comment)

[Alphayeeeet]

Already resollved by upgrading to 8.12.x.

[Alphayeeeet]

Reopen because of not enough features:
#6014 #8441

ETCD integration in Fleet UI needs option to specify paths for serving certificates (Client-Cert/Key and serving CA).
For now it is only possible to monitor non TLS protected etcd instances.

Kubernetes provides etcd-certificates to use on host-fs (/etc/kubernetes/static-pod-resources/...). Integration should use those paths including node name for getting correct peer certificates.

[Alphayeeeet]

Btw. perhaps consider opening a new issue to support trusted ca-certificates in Kubernetes kube-controller-manager and kube-scheduler config. (The option to set verification mode is available, but ca-certificates are missing.

[Alphayeeeet]

Would be resolved by #9414

[Alphayeeeet]

Should be resolved