Improve Kubernetes APIserver metrics integration

We use the Elastic agent to monitor our Serverless infrastructure and - in general - all kinds of Kubernetes clusters deployed in Elastic through the different CSPs. We have seen a gap in the metrics gathered by the Kubernetes module in the Elastic Agent.
The metrics that we miss are related to the [kubernetes.apiserver](https://docs.elastic.co/integrations/kubernetes/kube-apiserver) dataset.
We are trying to monitor the etcd status, which metrics are published through the Kubernetes API server in the CSP-managed environments. We want to check the size and the latency of the requests but we only have one metric, the [kubernetes.apiserver.etcd.object.count](https://arc.net/l/quote/vszuuskh).
If we query the API server, we can see other interesting etcd metrics that we are interested in:

- **etcd size**: `apiserver_storage_size_bytes`
- **etcd request duration**: `etcd_request_duration_seconds_count` and `etcd_request_duration_seconds_bucket`
- **etcd number of requests**: `etcd_requests_total`

Ideally, it would be great to have the ability to customize freely which fields you want to gather from the Kubernetes API server since there are a lot and different use cases that have different needs.

Cc: @mlunadia