Add monitoring stats to mapping script

[carsonip]

Motivation/summary

Add a script that modifies various files in-place to contain mappings generated from apm-server stats read from stdin.

Example usage: python3 script/stats_to_mapping.py ~/projects/beats/metricbeat/module/beat/_meta/fields.yml ~/projects/beats/metricbeat/module/beat/stats/_meta/fields.yml ~/projects/elasticsearch/x-pack/plugin/core/template-resources/src/main/resources/monitoring-beats.json ~/projects/elasticsearch/x-pack/plugin/core/template-resources/src/main/resources/monitoring-beats-mb.json ~/projects/integrations/packages/elastic_agent/data_stream/apm_server_metrics/fields/beat-fields.yml < apm-server.stats.json

Checklist

• Update CHANGELOG.asciidoc
• Documentation has been updated

For functional changes, consider:

• Is it observable through the addition of either logging or metrics?
• Is its use being published in telemetry to enable product improvement?
• Have system tests been added to avoid regression?

How to test these changes

Related issues

Part of #13475

[mergify]

This pull request does not have a backport label. Could you fix it @carsonip? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-7.17 is the label to automatically backport to the 7.17 branch.
• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.

NOTE: backport-skip has been added to this pull request.