[Infra] Improve /api/metrics/source endpoint

[crespocarlos]

Summary

Infra uses /api/metrics/source/{sourceId} endpoint to register, update, and retrieve the Infra Settings. However, it also returns a representation of a data view, built from the Metrics Indices config, which is used by the Infra UIs.

The idea is to split that process, following the way Logs UI handles the same idea, making the /source endpoint only responsible for managing the Saved Object that stores the settings config, removing the retrieval of the indices fields

e.g: response

{
  "source": {
    "id": "default",
    "version": "WzM0LDFd",
    "updatedAt": 1712829507652,
    "origin": "stored",
    "configuration": {
        "name": "Default",
        "description": "",
        "metricAlias": "remote_cluster:metrics-*,remote_cluster:metricbeat-*",
        "inventoryDefaultView": "0",
        "metricsExplorerDefaultView": "0",
        "anomalyThreshold": 50
    }
  }
}

Probably depends on #180689

AC

• GET /api/metrics/source/{sourceId} doesn't return status attribute, which contains the indices fields

[elasticmachine]

Pinging @elastic/obs-ux-infra_services-team (Team:obs-ux-infra_services)

[neptunian]

Related: In #164094, we created a way to access the metrics indices without having to use this endpoint (/api/metrics/source/) by creating the metrics data access plugin and storing the indices value in metrics-data-source saved object. The indices are meant to stop being set in infrastructure-ui-source saved object as part of #168282. You can already get just the indices value by using client in the plugin https://github.com/elastic/kibana/tree/main/x-pack/plugins/metrics_data_access.