apmpackage: prepare for move to apm-data plugin

[axw]

Motivation/summary

This is a precursor to #12066.

The undocumented, best-effort 7.x migration data stream migration pipeline has been removed. To migrate, users can install an older version and perform a stepped upgrade.

We add mappings for various fields to the integration package that were previously unmapped.

The following fields are all newly mapped as flattened:

• error.custom
• error.exception.attributes
• error.exception.stacktrace
• error.log.stacktrace
• http.request.body (always send as a JSON object with a single field, original)
• http.request.env
• http.request.cookies
• span.message.headers
• span.stacktrace
• transaction.custom
• transaction.message.headers

http.{request,response}.headers have been changed from object to flattened. Even though these were mapped as object type, they were inheriting dynamic: false and thus the headers never got mapped. The result of this change is headers will be returned in fields now, as http.request.headers: {Content-Length: ...}. There's no change to the _source for these fields.

http.response.headers_sent is mapped as a boolean.
transaction.span_count.started is mapped as a long for consistency with transaction.span_count.dropped.

The following fields are mapped as keyword, in line with the dynamic mapping rules of the apm-data plugin's templates:

• span.db.instance
• span.db.type
• span.db.user.name
• span.db.statement
• span.message.body
• span.message.routing_key
• transaction.message.body
• transaction.message.routing_key

Checklist

• Update CHANGELOG.asciidoc
• Update package changelog.yml (only if changes to apmpackage have been made)
  - [ ] Documentation has been updated

How to test these changes

• Check that error/exception stack traces show up correctly in APM UI
• Check that HTTP request/response headers show up in APM UI
• Check that SQL statements show up in APM UI

Related issues

#12066

[mergify]

This pull request does not have a backport label. Could you fix it @axw? 🙏
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.

[simitt]

Verified with BC-2 that

• error stacktraces show up
• http headers show up
• sql statements show up as expected