Fix incorrect timeout status code in trace produced by self instrumentation

[carsonip]

Motivation/summary

WIP

TODO: implement the actual fix. Although trace transaction result is HTTP 5xx, it is not the 5xx returned by timeout middleware, as timeout middleware should return 503. It is actually the one from writeStreamResult processStreamError default HTTP 500:

apm-server/internal/beater/api/intake/handler.go

Line 148 in 52849ff

errID := request.IDResponseErrorsInternal

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

Fixes #14232

[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.
• backport-8.x is the label to automatically backport to the 8.x branch.

[mergify]

backport-8.x has been added to help with the transition to the new branch 8.x.
If you don't need it please use backport-skip label.

[1pkg]

closing this in favor of #15123 and #15122