Add format to transaction.duration.us

[simitt]

Motivation/summary

Adds input- and output-format and precision to transaction.duration.us for improved usability and

What's controversal here is that we format a field that has us in its name to milliseconds.

Checklist

- [ ] I have signed the Contributor License Agreement.

• My code follows the style guidelines of this project (run make check-full for static code checks and linting)
• I have rebased my changes on top of the latest master branch

- [ ] I have made corresponding changes to the documentation
- [ ] I have added tests that prove my fix is effective or that my feature works

• New and existing unit tests pass locally with my changes

• I have updated CHANGELOG.asciidoc
• needs update in Kibana

How to test these changes

1. Check out a local copy of Kibana and update the index pattern to contain the new format; start Kibana and add APM data
2. navigate through APM UI and check out implications on Kibana data for transaction.duration.us.

Related issues

implements #3751

[simitt]

Actually we have specifically removed the format from the field previously (#1717) to avoid confusion around showing duration from a us field as milliseconds. So most probably we should not apply this change here and rather close the PR.

[apmmachine]

💚 Build Succeeded

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Build stats

• Build Cause: [Branch indexing]

• Start Time: 2020-05-08T18:51:48.483+0000

• Duration: 48 min 12 sec (2892455)

• Commit: b0b32fc

Test stats 🧪

Test	Results
Failed	0
Passed	3222
Skipped	145
Total	3367

Steps errors

Expand to view the steps failures

• Name: Test Sync

  • Description: ./script/jenkins/sync.sh

  • Result: FAILURE

  • Duration: 3 min 51 sec

  • Start Time: 2020-05-08T19:02:46.329+0000

  • log

[simitt]

Closing this out based on discussions in the related issue #3751