[Feature] Timeslice metric indicator for elasticstack_kibana_slo (supporting percentile aggregations)

[irregulator]

Hi! Thanks for developing this Terraform provider!

Describe the resource you would like to have implemented.

We would like to manage elasticstack_kibana_slo objects with Terraform and employ Timeslice metric indicator. We are particularly interested in Percentile aggregation. This feature is already present in Kibana as described in:

• https://www.elastic.co/guide/en/observability/current/slo-create.html#timeslice-metric-sli
• https://www.elastic.co/docs/api/doc/kibana/v8/operation/operation-createsloop#operation-createsloop-body-application-json-elastic-api-version-2023-10-31-slos_indicator_properties_timeslice_metric-indicator

Describe the solution you'd like

A new indicator could look like this:

timeslice_custom_indicator {
  index           = "heartbeat-*"
  filter          = "monitor.name: \"loadbalancer\""
  timestamp_field = "@timestamp"
  metrics = [
    {
      name = "p95"
      field = "http.rtt.total.us"
      aggregation = "percentile"
      percentile = 95
    } 
  ]  
  equation = "p95"
  comparator = "LT"
  threshold = 2000000
}

Describe alternatives you've considered

None

Additional context

Here's the equivalent API request when creating an example latency SLO with Timeslice metrics and Percentile aggregation through Kibana UI:

{
  "name": "Latency SLO",
  "description": "Latency SLO",
  "indicator": {
    "type": "sli.metric.timeslice",
    "params": {
      "index": "heartbeat-*",
      "filter": "monitor.name: \"loadbalancer\"",
      "metric": {
        "metrics": [
          {
            "name": "A",
            "aggregation": "percentile",
            "field": "http.rtt.total.us",
            "filter": "",
            "percentile": 95
          }
        ],
        "equation": "A",
        "comparator": "LT",
        "threshold": 2000000
      },
      "timestampField": "@timestamp"
    }
  },
  "budgetingMethod": "timeslices",
  "timeWindow": {
    "duration": "30d",
    "type": "rolling"
  },
  "objective": {
    "target": 0.99,
    "timesliceTarget": 0.95,
    "timesliceWindow": "1m"
  },
  "tags": [],
  "groupBy": "url.domain"
}