[EPM] Reassess default index template priority

[skh]

Currently, we use the following priorities for built-in or generated index templates:

• 100 for the built-ins metrics-*-*, logs-*-*, and synthetics-*-*, those come from elasticsearch, see https://www.elastic.co/guide/en/elasticsearch/reference/current/index-templates.html
• 200 for generated index templates for data streams of integrations (see [EPM] Conditionally generate ES index pattern name based on dataset_is_prefix #89870)
• 150 for generated index templates for data streams of integrations when dataset_is_prefix is true (also see [EPM] Conditionally generate ES index pattern name based on dataset_is_prefix #89870)

The priority 200 might conflict with what we suggest people do in the elasticsearch documentation linked above:

For example, if you don’t use the Elastic Agent and want to create a template for the logs-* index pattern, assign your template a priority of 200. This ensures your template is applied instead of the built-in template for logs--.

This issue is to decide if we want to use a different default priority, change the recommendation in the docs, or leave everything as it is now.

[elasticmachine]

Pinging @elastic/fleet (Feature:EPM)

[ph]

@ruflin I would be tented to just update the doc to give us more room if we have more thing to changes, I compare this to z-index problems, maybe 500-1000 would be a good level.

[ruflin]

I wonder if the ES team has an opinion on this @jpountz @dakrone ?

[dakrone]

I don't have a strong opinion about this, but for the sake of priority consistency between versions, I'd prefer updating the documentation as the solution.

[skh]

I've opened elastic/elasticsearch#68982 for a potential documentation change.

[skh]

Docs were changed in elastic/elasticsearch#69006