Closes elastic#220815

## Summary

This PR adds telemetry for _ignored fields. The goal is to have an alert
based on this field and get notified in case a user hits the mapping
limit, but by default, we don't expect this to happen.

This PR adds `count_ignored_fields_by_rule_type` field that counts the
number of _ignored fields per rule type.

In the future, we can extend the telemetry data to also include the
actual number of mappings over the limit (there is a [feature
request](elastic/elasticsearch#68947) for
adding field count information to index API)

### How to test

- Add a lot of dynamic fields as mentioned here:
elastic#216719
- Create a rule with a custom threshold rule with multiple group by
fields to generate an alert with _ignored field
- Run the following API and check the value of
`count_ignored_fields_by_rule_type`
  ```
  POST kbn:/internal/telemetry/clusters/_stats?apiVersion=2
  {
    "unencrypted": true,
    "refreshCache": true
  }
  ```

<details>
<summary> Here is what it looks like:</summary>

![image](https://github.com/user-attachments/assets/bb71bde1-9002-4f96-9b84-3e4c7b6f0aed)

![image](https://github.com/user-attachments/assets/725fda41-454b-4ed0-a2dc-40796729bc19)

</details>

---------

Co-authored-by: kibanamachine <42973632+kibanamachine@users.noreply.github.com>
(cherry picked from commit cde7a86)

# Conflicts:
#	x-pack/platform/plugins/shared/alerting/server/usage/lib/get_telemetry_from_alerts.test.ts