[Infra] Change host count KPI query

[crespocarlos]

Summary

Hosts count KPI in the hosts view has to count only hosts collected by system module.

"should": [
  {
    "term": {
      "event.module": "system"
    }
  },
  {
    "term": {
      "metricset.module": "system"
    }
  }
],
"minimum_should_match": 1

We could create an endpoint an run the query in the server instead of doing in it on the client side https://github.com/elastic/kibana/blob/main/x-pack/plugins/observability_solution/infra/public/pages/metrics/hosts/hooks/use_host_count.ts#L30

Acceptance Criteria

• Only hosts monitored with system integration are accounted for

[elasticmachine]

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