Add telemetry for ignored fields

[maryam-saeidi]

Closes #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 for adding field count information to index API)

How to test

• Add a lot of dynamic fields as mentioned here: Auto increase fields limit of the alert indices #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
  }
  

Here is what it looks like:

[elasticmachine]

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[maryam-saeidi]

Closing in favor of #221480