Include field name in decoding errors

[jalvz]

Motivation/summary

Troubleshooting decoding errors is currently very hard because the error doesn't include the field name, and there are many call sites (and calls) that can return an error.

This adds the field name to the decoding error message.

Checklist

• I have signed the Contributor License Agreement.
• I have updated CHANGELOG.asciidoc

I have considered changes for:

• documentation
• logging (add log lines, choose appropriate log selector, etc.)
• metrics and monitoring (create issue for Kibana team to add metrics to visualizations, e.g. Kibana#44001)
• automated tests (add tests for the code changes, all unit tests pass locally)
• telemetry
• Elasticsearch Service (https://cloud.elastic.co)
• Elastic Cloud Enterprise (https://www.elastic.co/products/ece)
• Elastic Cloud on Kubernetes (https://www.elastic.co/elastic-cloud-kubernetes)

How to test these changes

make test

[apmmachine]

💚 Build Succeeded

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Build stats

• Build Cause: [Pull request #3866 updated]

• Start Time: 2020-06-11T06:35:41.026+0000

• Duration: 47 min 49 sec

Test stats 🧪

Test	Results
Failed	0
Passed	3210
Skipped	146
Total	3356

Steps errors

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• Name: Test Sync

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

  • Duration: 3 min 44 sec

  • Start Time: 2020-06-11T06:47:28.044+0000

  • log

[axw]

Can you give an example of an error where this would have helped? I'm wondering why we're getting these "fetch" errors at all - shouldn't JSON Schema validation have prevented them?

[jalvz]

@axw they are relevant basically for development. One example for instance when you get it is if you call the wrong decoder method (eg. use decoder.String instead of decoder.StringPtr, etc)

[axw]

@axw they are relevant basically for development. One example for instance when you get it is if you call the wrong decoder method (eg. use decoder.String instead of decoder.StringPtr, etc)

OK. So in this case it would be like the schema said something is optional, but in the code we made it required by using the wrong method. Fair enough.

[codecov-commenter]

Codecov Report

Merging #3866 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3866   +/-   ##
=======================================
  Coverage   79.80%   79.80%           
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  Files         135      135           
  Lines        6234     6234           
=======================================
  Hits         4975     4975           
  Misses       1259     1259