docs: Document custom field mappings

[colleenmcginnis]

Preview: View the Elasticsearch index template

Motivation/summary

Adds more information to View the Elasticsearch index template including:

• How to add custom field mappings and at least one practical example
• How to add runtime fields and at least one practical example

@bmorelli25 these ☝️ are the topics included in the initial issue, but should we also add the following while we're updating this page?

• How to add/update metadata (in the Logistics step) and at least one practical example
• How to use dynamic templates and at least one practical example
• Available advanced options and when to use them
• How to add aliases and at least one practical example

Also, how far back should we backport?

Checklist

• @colleenmcginnis create initial outline
• @bmorelli25 initial review of the approach + answer the questions above
• @colleenmcginnis address initial feedback
• @elastic/apm-server review
• @colleenmcginnis address feedback, merge

Related issues

Closes elastic/observability-docs#2206

[bmorelli25]

My thoughts:

How to add/update metadata (in the Logistics step) and at least one practical example

I suppose we could add something here about how you can add arbitrary version numbers and metadata if they help you outside of the product. I'd be more tempted to just add a quick sentence or two and link to this page for more details: https://www.elastic.co/guide/en/elasticsearch/reference/8.11/indices-component-template.html#component-templates-metadata

How to use dynamic templates and at least one practical example

If we had a good example of how our users might use this feature, then it would be very easy to add something here.
@elastic/apm-server any recommendations on how someone might use dynamic mappings with APM?

Available advanced options and when to use them

I don't know enough about this and the potential it does/doesn't have to mess up the APM app if folks touch these settings

How to add aliases and at least one practical example

Similar response from me here.

cc @axw for the tech lead perspective on these questions

[axw]

I can't think of any common use cases related to the APM data streams/index templates for dynamic templates, advanced mapping options, or aliases.

[simitt]

I can't think of any common use cases related to the APM data streams/index templates for dynamic templates, advanced mapping options, or aliases.

++
Is there a specific request to add docs for these? Based on previous user questions, I'd say that describing how to customize component templates for adding field mappings and how to change retention periods is the most critical piece (don't remember anyone asking for dynamic templates).