publish: implement BatchProcessor; remove tracing

[axw]

Motivation/summary

• Modify publish.Publisher so that it implements model.BatchProcessor directly.
• Remove the tracing instrumentation from publisher. We don't use and don't support libbeat processors, so this is not useful.

Checklist

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

How to test these changes

1. Enable instrumentation
2. Send some events to apm-server
3. Check that no "Publisher" type transactions are traced

Related issues

Related to #6002

[apmmachine]

💚 Build Succeeded

the below badges are clickable and redirect to their specific view in the CI or DOCS

Expand to view the summary

Build stats

• Start Time: 2021-10-04T06:16:49.202+0000

• Duration: 50 min 43 sec

• Commit: 655e6a8

Test stats 🧪

Test	Results
Failed	0
Passed	6141
Skipped	14
Total	6155

🤖 GitHub comments

To re-run your PR in the CI, just comment with:

• /test : Re-trigger the build.

• /hey-apm : Run the hey-apm benchmark.

• /package : Generate and publish the docker images.

[mergify]

This pull request is now in conflicts. Could you fix it @axw? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b publish-modelprocessor upstream/publish-modelprocessor
git merge upstream/master
git push upstream publish-modelprocessor

[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.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.

[marclop]

Tested with 7.16.0 BC2 locally:

1. ./apm-server -E output.elasticsearch.username=admin -E output.elasticsearch.password=changeme -E instrumentation.enabled=true -e
2. Verify that no results are returned with the following query:

GET apm-*,traces-apm*,logs-apm*/_search
{
  "query": {
    "bool": {
      "should": [
        {
          "match": {
            "transaction.type": "Publisher"
          }
        },
        {
          "match": {
            "span.type": "Publisher"
          }
        }
      ]
    }
  }
}