All of our overarching developer docs issues are kind of old (#4059 was a bit dated so we decided to close it) so @jsoriano And I decided to open a new meta-issue, as I in particular had some problems with the Beats developer guide before I got here.

• The Guide briefly mentions _meta/data.json once in the context of it being an autogenerated file, but I can't find any further information on what it does, why it's there, etc. [metricbeat] Expand metricbeat dev guide for testing #12105
• make fmt should probably be mentioned, as CI will test this.
• The testing section for adding a protocol to packetbeat needs to be filled. May be covered by Update 'new protocol guide' #1111
• New Beat docs updates [Docs] Update the API used in docs/devguide/newbeat.asciidoc #7159
• Beats testing guide Documents beats testing for developers #2052 PR: [metricbeat] Expand metricbeat dev guide for testing #12105
• More complete documentation of fields.yml Document fields.yml #6049
  Older issues that need looking at:
• Developer Documentation Update - Beater/lsbeat #6093
• Missing mention of module/{module}/module.yml file in the developper guide #5658
• Publisher Pipeline #4598 has an uncompleted item for developer docs

Metricbeat specific:

• A best practices document for metricsets, to standardize things like test layout, error handling, etc.
• Data transformation for metricsets is mentioned in two places, here and here. The first page demonstrates a typical example of a data.go file, but data.go itself and usage of the standard eventMapping() function is explicitly called out in the second page. Maybe this should all be in one place?
• We do mention testing for metricsets but we don't mention the standard TestData() and TestFetch() integration tests that most metricsets have. [metricbeat] Expand metricbeat dev guide for testing #12105
• I've been here for a couple months and I'm still learning about new targets for make/mage. We should document all these somewhere?