Upgrade go-docappender to v2

[lahsivjar]

Motivation/summary

go-docappender/v2 fixes a crucial bug with data race: elastic/go-docappender#162

It also has some other fixes though they don't apply to apm-server since apm-server doesn't have document retries enabled:

• Fix allocation the document retry buffer go-docappender#160
• fix: update elastic.events.queued with the correct number of queued events go-docappender#150

In addition, it also exposes a new metric for elasticsearch flushed bytes: elastic/go-docappender#161

Checklist

• Update CHANGELOG.asciidoc
  - [ ] Documentation has been updated

How to test these changes

• Don't pass a reused bytes.Buffer to the ES client go-docappender#162 is tested via test cases.
• feat: add uncompressed flushed bytes metric go-docappender#161 can be tested by checking for existence of elasticsearch.flushed.uncompressed.bytes metric.
• Other fixes don't need to be tested as they don't apply for apm-server use cases.

Related issues

N/A

[mergify]

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

[lahsivjar]

[For reviewers] I have added changelog entries to APM-Server for go-docappender fixes so that they can be tested and documented. Let me know if this doesn't sound like a good idea.