[Discover] Document metrics grid aggregations

Request to document changing aggregation defaults in Metrics in Discover.

Page to update: https://www.elastic.co/docs/solutions/observability/infra-and-hosts/discover-metrics

Metrics in Discover applies a default aggregation per metric type. Users can now change that default from the UI, without editing the ES|QL query. A **Configuration** flyout (Aggregations) lets them pick a different aggregation:

- **Counter**: Average, Sum (default), Minimum, Maximum
- **Gauge**: Average (default), Sum, Minimum, Maximum
- **Histogram**: 50th percentile, 75th percentile, 90th percentile, 95th percentile (default), 99th percentile

Available in Serverless and Elastic Stack 9.6+ once enabled.

Related:
- https://github.com/elastic/kibana/issues/265215
- https://github.com/elastic/kibana/issues/288597 (blocked until the feature is enabled for all users)

Contacts:
- PM: @miguel-sanchez-elastic
- Engineering: @kpatticha

Update the Discover metrics page at https://www.elastic.co/docs/solutions/observability/infra-and-hosts/discover-metrics. Start by reading the existing page and compare it with the requested Configuration flyout behavior. Done means documenting the selectable aggregations for counters, gauges, and histograms, including defaults and availability in Serverless and Elastic Stack 9.6+ once enabled.

Written by the indexing model from the issue text.