add span.id to documents with transaction.id

[SylvainJuge]

Motivation/summary

Adds span.id for the documents that already have a transaction.id in order to allow referencing transactions AND spans by using a single span.id field.

Checklist

• Update CHANGELOG.asciidoc
• Update package changelog.yml (only if changes to apmpackage have been made)
• 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

depends on elastic/apm-data#112

Related issues

elastic/apm-data#82

[mergify]

This pull request does not have a backport label. Could you fix it @SylvainJuge? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-7.x is the label to automatically backport to the 7.x branch.
• backport-7./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[apmmachine]

💔 Build Failed

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Build stats

• Start Time: 2023-07-12T16:00:34.220+0000

• Duration: 11 min 39 sec

Steps errors

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Load a resource file from a library

• Took 0 min 0 sec . View more details here
• Description: approval-list/elastic/apm-server.yml

Run benchmarks

• Took 2 min 24 sec . View more details here
• Description: ./.ci/scripts/bench.sh

Recursively delete the current directory from the workspace

• Took 0 min 1 sec . View more details here
• Description: script returned exit code 2

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[SylvainJuge]

Closing this as another approach was taken in elastic/apm-data#112 which does not requires changes in apm-server by modifying only the intake parts (both otlp and elastic).